The Structure and Classification of the...

STRUCTURE AND CLASSIFICATION OF THE AIITHHOPODA. 523

The Structure and Classification of theArthropoda.

ByE. Ray Lankcster, M.A., LX.D., F.R.S.,

Director of the Natural History Departments of the British Museum.

With PUe 42.

[BY the great kindness of the proprietors of the tenth edi-tion of the 'Encyclopaedia Britannica' I have received per-mission to reprint in this journal the articles ARTHROPODAand ARACHNIDA, which I contributed to its pages. I havebeen anxious that morphologists should consider the viewswhich I have put forward in these articles (written now nearlyfour years ago). At the same time I have observed that theyhave entirely escaped the notice of two authors who haverecently written general essays on the Arthropoda, viz. Dr.A. S. Packard, of Salem, Mass., and Mr. G. H. Carpenter,of Dublin. I have revised both articles only in regard toverbal inaccuracies, excepting where I have definitely statedthat new matter is introduced. I hope that in their presentform these articles will not fail to come under the notice of

s.—E. R. L.]

ARTHROPODA is the name of one of the three sub-phylainto which one of the great phyla (or primary branches) of

524 E. EAY LANKBSTER.

coeloinoccelous animals—the Appendiculata—is divided, theother two being respectively the Ch^topoda and the Rotifera.The word " Arthropoda" was first used in classification bySiebold and Stannius (' Lehrbuch der vergleich. Auatomie/Berlin, 1845) as that of a primary division of animals, theothers recognised in that treatise being Protozoa, Zoophyta,Vermes, Mollusca, and Vertebrata. The names Oondylopodaand Gnathopoda have been subsequently proposed for thesame group. The word refers to the jointing of the chitinisedexo-skeleton of the limbs or lateral appendages of the animalsincluded, which are, roughly speaking, the Crustacea, Arach-nida, Hexapoda (so-called " t rue insects"), Centipedes, andMillipedes. This primary group was set up to indicate theresiduum of Cuvier's Articnlata when his class Annelides(the modern Cheefcopoda) was removed from that " embranche-ment." At the same time Siebold and Stannius renovatedthe group Vermes of• Linuasus, and placed in it the ChEefcopodsand the parasitic worms of Cuvier, besides the Rotifers andTurbellarian worms.1

1 As a matter of fact the group Arthropoda itself, thus constituted, wasprecisely identical in its area with the class Insecta of Linnaeus, the Entomaof Aristotle. But by causes which it is not easy to trace the word "Insect"had become limited since the days of Linnaeus to the Hexapod Pterygoteforms, to the exclusion of his Aptera. Lamarck's penetrating genius is chieflyresponsible for the shrinkage of the word Insecta, since it was lie who, fortyyears after Linnseus's death, set up and named the two great classes Crustaceaand Araclmida (included by Linnseus under Insecta as the order "Aptera")assigning to them equal rank with the remaining Insecta of Linneeus, forwhich he proposed the very appropriate class-name " Hexapoda." Lamarck,however, appears not to have insisted on this name Hexapoda, and so theclass of Pterygote Hexapods came to retain the group-name Insecta, whichis, historically or etymological ly, no more appropriate to them than it is tothe classes Crustacea and Araclmida. The tendency to retain the originalname of an old and comprehensive group for one of the fragments into whichsuch group becomes divided by the advance of knowledge—instead of keepingthe name for its logical use as a comprehensive term, including the newdivisions, each duly provided with a new name—is most curiously illustratedin the history of the word Physiology. Cicero says, "Physiologia naturaeratio," and such was the meaning of the name Physiologus, given to a

STOUCTGBE AND CLASSIFICATION OF THE AllTHROPODA. 525

The result of the knowledge gained in the last quarter ofthe nineteenth century has been to discredit altogether thegroup Vermes, thus set up and so largely accepted by Germanwriters even at the present day. We have, in fact, returnedvery nearly to Cuvier's conception of a great division or branch,which he called Avticnlata, including the Arthropoda and theChtetopoda (the latter equivalent to the Annelides of Lamarck,a name adopted by Guvier), and differing from it only by theinclusion of the Rotif era. The name Articulata, introduced byCuvier, has not been retained by subsequent writers. Thesame, or nearly the same assemblage of animals has beeu calledEntomozoariaby DeBlaiuville (1882), ArthrozoabyBurmeister(1843), Entornozoa or Anuellata by Milne-Edwards (1855),and Annulosa byM'Leay (1819), who was followed by Huxley(1856). The character pointed to by all these terms is thatof a ring-like segmentation of the body. This, however, isnot the character to which we now ascribe the chief weightas evidence of the genetic affinity and monophyletic (uni-ancestral) origin of the Chastopods, Rotifers, and Arthropods.It is the existence in each ring of the body of a pair of hollowl a t e r a l a p p e n d a g e s or pa rapod ia , moved by intrinsicmuscles and penetrated by blood-spaces, which is the leadingfact indicating the affinities of these great sub-phyla, anduniting them as blood relations. The pai-apodia (fig. 7) ofthe marine branchiate worms are the same things geneticallyas the " l egs " of Crustacea and insects (fags. 9 and 10).Hence the term Appendiculata was introduced by Lankester

cyclopaedia of wliat was kuown and imagined about earth, sea, sky, birds,beasts, and fishes, which for a thousand years was the authoritative source ofinformation on these matters, and was translated into every European tongue.With the revival of learning, however, first one and then another special studybecame recognised—anatomy, botany, zoology, mineralogy, until at last thegreat comprehensive term Physiology was bereft of all its once-includedsubject-matter excepting the study of vital processes pursued by the morelearned members of the medical profession. Professional tradition, and anastute perception on iheir part of the omniscience suggested by the terms,have left the medical men in English-speaking lands in undisturbed butillogical possession of the words physiology, physic, and physician.

526 E. RAY LANKESTER.

(preface to the English edition of Gegenbaur's ' ComparativeAnatomy,' 1878) to indicate the group. The relationships ofthe Arthropoda thus stated are shown in the subjoined table :

f Sub-phylum 1. Rotifera.Phylum APPENDICDLATA^ „ 2. Chsetopoda.

I ,, 3. Arthropoda.

The Rotifera are characterised by the retention of whatappears in Molluscs and Chaatopods as an embryonic organ,the velum or ciliated prasoral girdle, as a locomotor andfood-seizing apparatus, and by the reduction of the muscularparapodia to a rudimentary or non-existent condition in allpresent surviving forms except Peda l ion . In many im-portant respects they are degenerate—reduced both in sizeand elaboration of structure.

The Chastopoda are characterised by the possession ofhorny epidermic chsetas embedded in the integument andmoved by muscles. Probably the chaatse preceded thedevelopment of parapodia, and by their concentration, andthat of the muscular bundles connected with them at thesides of each segment, led directly to the evolution of theparapodia. The parapodia of Chtetopoda are never coatedwith dense chitin, and are, therefore, never converted intojaws; the primitive "head-lobe" or prostomium persists,and frequently carries eyes and sensory tentacles. Further,in all members of the sub-phylum Chastopoda the relativeposition of the prostomium, mouth, and peristomium or firstring of the body retains its primitive character. We do notfind in Chastopoda that parapodia, belonging to primitivelypost-oral rings or body-segments (called " somites," asproposed by H. Milne-Edwards), pass in front of the mouthby adaptational shifting of the oral aperture. (See, how-ever, 8.)

The Arthropoda might be better called the " Gnathopoda/'since their distinctive character is that one or more pairs ofappendages behind the mouth are densely chitinised andturned (fellow to fellow on opposite sides) towards one

STBnCTUBB AND CLASSIFICATION OF THE ARTHROPODA. 527

auother so as to act as jaws. This is facilitated by animportant general change in the position of the parapodia;their basal attachments are all more ventral in position thanin the Chtetopoda, and tend to approach from the two sidestowards the mid-ventral line. Very usually (but not in theOnychophora = Peripatus) all the parapodia are platedwith chitin secreted by the epidermis, and divided into aseries of joints—giving the " arthropodous" or hingedcharacter.

There are other remarkable and distinctive features ofstructure which hold the Arthropoda together, and render itimpossible to conceive of them as having a polyphyleticorigin,—that is to say, as having originated separately by twoor three distinct lines of descent from lower animals; and, onthe contrary, establish the view that they have been deve-loped from a single line of primitive Gnathopods which aroseby modification of parapodiate annulate worms not veryunlike some of the existing ChEetopods. These additionalfeatures are the following:—(1) All existing Arthropodahave an ostiate heart and have undergone " phleboedesis,"that is to say, the peripheral portions of the blood-vascularsystem are not fine tubes as they are in the Cheetopoda andas they were in the hypothetical ancestors of Arthropoda,but are swollen so as to obliterate to a large extent thecoelom, whilst the separate veins entering the dorsal vesselor heart have coalesced, leaving valvate ostia (see Pig. 1*) bywhich the blood passes from a pericardial blood-sinus formedby the fused veins into the dorsal vessel or heart (seeLankester's 'Zoology,' part ii, introductory chapter; A.and 0. Black, 1900). The only exception to this is in thecase of minute degenerate forms where the heart has dis-appeared altogether. The rigidity of the integument causedby the deposition of dense chitin upon it is intimatelyconnected with the physiological activity and form of all theinternal organs, and is undoubtedly correlated with the totaldisappearance of the circular muscular layer of the body-wallpresent in ChEetopods. (2) In all existing Arthropoda the

528 B. BAY LANKBSTEK.

region in front of the mouth is no longer formed by theprimitive prostomium or head-lobe, but one or more seg-ments, originally post-oral, with their appendages havepassed in front of the mouth (prosthonieres). At the sametime the prostomium and its appendages cease to be recog-nisable as distinct elements of the head. The brain nolonger consists solely of the nerve-ganglion mass proper tothe pi'ostomial lobe, as in Cbasfcopoda, but is a composite(syncerebrum) produced by the fusion of this and the nerve-ganglion masses proper to the prosthomeres or segmentswhich pass forwards, whilst their parapodia ( = appendages)

irFIG. 1*.—Diagram to show the gradual formation of the Arthropod

pericardia! blood-sinus aud "ostiate" heart by the swelling up(phlebcedesis) of the veins entering the dorsal vessel or heart of aChtetopod-like ancestor. The figure on the left represents thecondition in a Cheetopod, that on the right the condition in anArthropod; the other two are hypothetical intermediate forms.(After Lankester, 'Quart. Journ. Micr. Sci.,' vol. xxxiv, 1893.)

become converted into eye-stalks and antennas, or morerarely grasping organs. (3) As in Chsetopoda, ccelomicfunnels (ccelomoduets) may occur right and left as pairs ineach ring-like segment or somite of the body, and some ofthese are in all cases retained as gonoducts and often asrenal excretory organs (green glands, coxal glands ofArachnida—not crural glands, which are epidermal inorigin); but true nephridia, genetically identical with thenephridia of earthworms, do not occur (on the subject of

STBUCTCJJRB AND CLASSIFICATION OP THI5 AllTUROPODA. '529

coelom, coeloruoducts, and nephridia, see the introductorychapter of part ii of Lankester's ' Treatise on Zoology').

Tabular Statement of the Grades, Classes, andSub-classes of the Arthropoda.—It will be convenientnow to give in the clearest form a statement of the largersubdivisions of the Arthropoda which it seems necessary torecognise at the present day. The justification of thearrangement adopted will form the substance of the restof the present article. The orders included in the varionsclasses are not discussed here, but are treated of underthe following titles:—PEKJPATUS (Onychophora), MYRIAPODA(Diplopoda and Chilopoda), ARAUHNIDA, INSJECTA (Hexapoda),and CBUSTACEA.

STJB-PHYLUM ARTHKOPODA (of the Phylum Appeu-diculata).

Grade A. Hyparthropoda (hypothetical forms connecting au-cestors of Chaetopoda with those of Arthropoda).

Grade B. Protarthropoda.

Class ONYCHOFHORA.Ex.—Peripatus.

Grade C. Euarthropoda.

Class 1. DJPLOPODA.

Ex.—Julus.

Class 2. AEACHNIDA.

Grade a. Anomomeristica.Ex.— Phacops.

Grade b. Nomomeristica.(a) Pantopoda.

Ex.—Pycnogonum.(b) Euarachmda.

Ex.—Limulus, Scorpio, Mygale , Aca-rus.

530 E. BAY LANKESTJEB.

Class 3. CRUSTACEA.

Grade a.—Entoniostraca.Ex.—Apus, B r a n c h i p u s , Cyclops , Ba lanus .

Grade b. Malacostraca.Ex.—Nebalia , A s t a c u s , Oniscus , Gam-

marus .Class 4. CHILOPODA.

Ex.—Scolopendra .Class 5. HBXAPODA (syn. I n s e c t a P t e r y g o t a ) .

Ex.—Locusta, Phryganea , Papi l io , Apis,Musca, Cimex, Lucanus , Machilis .

Incet'tsa sedis.—Tardigrada, Pentastomidas (degenerateforms).

The S e g m e n t a t i o n of t he Body of Ar th ropoda .—The body of the Arthropoda is more or less clearly dividediuto a series of rings, segments, or somites, which can beshown to be repetitions one of another, possessing identicalparts and organs which may be larger or smaller, modifiedin shape or altogether suppressed in one somite as comparedwith another. A similar constitution of the body is moreclearly seen in the Chsetopod worms. In the Vertebrata alsoa repetition of units of structure (myotomes, vertebrae, etc.) —which is essentially of the same nature as the repetition inArthropods and Chastopods, but in many respects subject topeculiar developments—is observed. The name "meta-merism " has been given to this structural phenomenon be-cause the " meres," or repeated units, follow one another inline. Each such " mere " is often called a " metamere."This is not the place in which to discuss the origin andessential nature of " metamerism " or " metameric segmenta-tion." Nevertheless a satisfactory consideration of thestructure of the Arthropoda demands a knowledge of whatmay be called the laws of metamerism. These are not sofully ascertained or formulated as might be expected. Therepetition of parts, which we note as metamerism, is, asHaeckel, Bateson, and others have recognised, only a special

STRUCTURE AND CLASSIFICATION OP THE ARTHROPODA. 531

case of a tendency of the organic body to repetition of struc-tural units or parts which, finds one expression in bilateralsymmetry. In certain worms (the Cestoidea and somePlanarians) metarneric segmentation is accompanied by theseparation of the completed metameres one by one from theolder (anterior) extremity of the chain (strobilation), but itby no means follows that inetameric segmentation has anecessary origin in such completion and separation of the" meres." On the contrary, metamerism seems to arise froma property of organisms which is sometimes more (eumero-genesis) and sometimes less (dysmerogenesis) fully exhibited,and in some groups not exhibited at all. Tlie most completeand, at the same time, simplest instances of metameric seg-mentation are to be seen in the larger Chsetopods, where somehundreds of segments succeed oue another—each practicallyindistinguishable in structure from the segment in front orfrom that behind; muscles, right and left appendage orparapodium, colour pattern of the skin, gut, blood-vessels,coelom, nephridia, nerve-ganglion, and nerves are preciselyalike in neighbouring segments. The segment which is leastlike the others is the first, for that carries the mouth and alobe projecting beyond it—the prostomium. If (as sometimeshappens) any of the hinder segments completes itself bydeveloping a prostomium, the chain breaks at that point, andthe segment which has developed a prostomium becomes thefirst or head-bearing segment of a new individual. Comparesuch an instance of metameric segmentation with that pre-sented by one of the higher Arthropods—e. g. the crayfish.Here the somites are not so clearly marked in the tegumentarystructures; nevertheless, by examining the indications givenby the paired parapodia, we find that there are twenty-onesomites present—a limited definite number which is also theprecise number found in all the higher Crustacea.

We can state as a FIRST LAW 1 of metamerism or somiteformation that it is either indefinite in regard to number of

1 The word "LATT" is used in this summary merely as a convenientheading for the statement of a more or less general proposition.

532 E. BAY LANKKS'J'BR.

metameres or somites produced or is definite. Animals in thefirst case we call anomomeristic; those in the second case,nomomeristic. The nornomeristic condition is a higher de-velopment, a. specialisation, of the anomomeristic condition.

The SECOND LAW, or generalisation, as to metamerismwhich must be noted is that the meres or somites (exceptingthe first with its prostomium) may be all practically alike, ormay differ from one another greatly by modification of thevarious constituent parts of the mere or somite. Metamerisedanimals are either hotncoomeric or heteromeric. The referenceto the variation in the form of the essential parts containedin a " metamere " or " somite " introduces us to the necessityof a general term for these constituent or subordinate parts;they may be called " meromes" (/aipog). The meromes pre-sent in a metamere or somite differ in different annulate orsegmented animals according to the general organisation ofthe group to which the animal belongs. As a matter of con-venience we distinguish in the Arthropod as meromes, first,the tegumentary chitinised plates called terga, placed on thedorsal aspect of the somites ; second, the similar sternal plates.In Chastopods we should take next to these the masses ofcircular and longitudinal muscular fibres of the body-wall andthe dorso-ventral muscles. The latter form the third sortof raerome present in the Arthropods. The fourth kind ofmerome is constituted by the parapodia or appendages; thefifth by the coelornic pouclies and their ducts and externalapertures (coelomo-ducts), whether renal or genital. Thesixth by the blood-vessels of the somite; the seventh by thebit of alimentary tract which traverses i t ; and the eighth bythe neuromere (nerve-ganglion pah', commissures, connectives,and nerve branches).

It becomes apparent from this enumeration that there area good many important elements or " meromes " in an Arthro-pod metamere or somite which can become the subject ofheteromerism, or, to use a more apt word, of " heterosis." Itis all the more necessary to insist upon this, inasmuch as thereis a tendency in the discussion of the segmentation of the

STRUCTURE AND CLASSIFICATION OF THE ARTHROPODA. 533

Arthropod body to rely exclusively upon the indications givenby the tegumentary chitinous plates and the parapodia.

The THIRD LAW of metamerism is that heteroraerism mayoperate in sucli a way as to produce definite regions of likemodification of the somites and their appendages, differingin their modification from that observed in regions before andbehind them. It is convenient to have a special word forsuch regions of like meres, and we call each a tagma (ray/ia,a regiment). The word " tagmosis" is applicable to theformation of such regions. In the Chaetopods tagmosisalways occurs to a small extent, so as to form the head. Insome Chtetopods, such as Chastopterus and the sedentaryforms, there is marked tagmosis, giving rise to three or evenmore tagmata. In Arthropods, besides the head, we findvery frequently other tagmata developed. But it is to benoted that in the higher members of each great class or lineof descent, the tagmosis becomes definite and characteristicjust as do the total number of meres or somites, whilst in thelower grades of each great class we find what may be regardedas varying examples of tentative tagmosis. The terms nomo-tagmic and anomotagmic are applicable with the same kindof implication as the terms nomomeristic and anomomeristic.

The FOURTH LAW of metamerism (auto-heterosis of themeroines) is that the meromes of a somite or series of somitesmay be separately and dissimilarly affected by heteromerism.It is common enough for small changes only to occur in theinner visceral meromes, whilst the appendages and terga orsterna are largely changed in form. But of equal importanceis the independent "heterosis" of these visceral meromeswithout any corresponding heterosis of the body-wall. Asinstances we may cite the gizzards of various earthworms,and the special localisation of renal, genital, and gastricmeromes, with obliteration elsewhere, in a few somites inArthropod a.

The FIFTH LAW, relating also to the independence of themeromes as compared with the whole somite, is the law ofautorhythmus of the meromes. Metamerism does not always

534 E. KAY LANKESTER.

manifest itself in the formation of complete new segments;but one merome may be repeated so as to suggest severalmetameres, whilst the remaining meromes are, so to speak,out of harmony with it and exhibit no repetition. Thus inthe hinder somites of the body of Apus the Crustacean wefind a series of segments corresponding apparently each to acomplete single somite, but when the appendages are examinedwe find that they have multiplied without relation to theother meromes of a somite; we find that the somites carryfrom two to seven pairs of appendages, increasing in numberas we pass backwards from the genital segment. The appen-dages are autorhythmic meromes in this case. They take ona quasi-independent metamerism, and are produced in numberswhich have no relation to the numbers of the body-rings,muscles, and neuromeres. This possibility of the inde -p e n d e n t metameric multiplication of a single merome musthave great importance in the case of dislocated meromes, andno doubt has application to some of the metameric phenomenaof Vertebrates.

A case which appears at first sight to be one of " auto-l'hythmus" of the parapodia is that of the Diplopods (Julus,etc.), in which each apparent somite carries two pairs of legsor parapodia. It looks at first as though this were due to theindependent multiplication of the legs ; but it is not. Con-trary to what obtains in Apus, we find in Julus that there isa well-marked somite in the embryo corresponding to eachpair of legs, and that the adult condition arises from a fusionof the tegumentary meromes of adjacent somites (see below,"Fusion").

The SIXTH LAW is the law of dislocation of meromes. Thisis a very important and striking phenomenon. A merome,such as a pair of appendages (Araneas) or a neuromere, or amuscular mass (frequent), may (by either a gradual or suddenprocess, we cannot always say which) quit the metamere towhich it belongs, and in which it originated, and pass byactual physical transference to another metamere. Frequentlythis new position is at a distance of several metameres from

STRUCTURE AND CLASSIFICATION OF THE ARTHBOPODA. 535

that to which the wandering naerome belongs in origin. Themovement is more usual from behind forwards than in thereverse direction ; but this pi'obably has no profound signifi-cance, and depends simply on the fact that, as a rule, the headmust be the chief region of development on account of itscontaining the sense organs and the mouth.

In the Vertebrata the independence of the meromes is morefnlly developed than in other metamerised animals. Not onlydo we get auto-heterosis of the meromes on a most extensivescale, but the dislocation of single meromes and of wholeseries (tagmata) of meromes is a common phenomenon. Thusin fishes the pelvic fins may travel forwards to a thoracic andeven jugular position in front of the pectoral fins; thebranchiomeromes lose all relation to the position of themeromes of muscular, skeletal, ccelomic, and nervous nature,and the heart and its vessels may move backwards from theiroriginal metameres in higher Vertebrates carrying nerve-loopswith them.

The SEVENTH LAW of metamerism is one which has beenpointed out to the writer by Mr. E. S. Goodrich, of MertonCollege, Oxford. It may be called the law of " translationof heterosis." Whilst actual physical transference of thesubstance of meromes undeniably takes place in such a caseas the passage of the pelvic fins of some fishes to the frontof the pectorals, and in the case of the backward movementof the opisthosomatic appendages of. spiders, yet the morefrequent mode in which an alteration in the position of aspecialised organ in the series or scale of metameres takesplace is not by migration of the actual material organ fromsomite to somite, but by translation of the q u a l i t y ormorphogenetic peculiarity from somite to somite accompaniedby correlative change in all the somites of the series. Thephenomenon may be compared to the transposition of a pieceof music to a higher or lower key. I t is thus that the lateralfins of fishes move up and down the scale of vertebral somites;1

and thus that whole regions (tagmata), such as those indicated1 Except in such cases as have just been cited.—U. R. L., 1904.

536 E. BAY LANKRSTER.

by the names cervical, thoracic, lumbar, and sacral, are trans-lated (accompanied by terminal increase or decrease in thetotal number of somites) so as to occupy differing numericalpositions in closely allied forms (cf. the varying number ofcervical somites in allied reptiles and birds).

What, in this rapid enumeration, we will venture to callthe EIGHTH LAW of metamerism is the law of homceosis, as itis termed by Bateson (1). Homoeosis is the making of amerome into the likeness of one belonging to another meta-mere, and is the opposite of the process of "heterosis"—already mentioned. We cite this law here because the resultof its opei'ation is to s imula te the occurrence of dislocationof meromes, and has to be carefully distinguished from thatprocess. A merome can and does, in individual cases ofabnormality, assume the form and character of the corre-sponding merome of a distant somite. Thus the antenna ofan insect has been found to be replaced by a perfectly well-formed walking leg. After destruction of the eye-stalk of ashrimp a new growth appears, having the form of an antenna.Other cases are frequent in Crustacea as individual abnor-malities. They prove the existence in the mechanism ofmetamerised animals of structural conditions which arecapable of giving these results. What those structural con-ditions are is a matter for separate inquiry, which we cannoteven touch here. It is not improbable that homoeosis ofdistant meromes may have given rise to permanent structuralchanges characteristic of whole groups of Arthropoda, sup-posing the abnormality once established to be favoured bynatural selection. Possibly the chelate condition of the prte-oral appendages of Arachnida maybe due to homoeosis trans-ferring the chelate form of post-oral limbs to what werepreviously antonniform rami.

We now come to the question of the production of newsomites or the addition of new somites to the series, and theconverse problem of the suppression of somites, whole orpartial. We state as the NINTH LAW of metamerism " thatnew somites or metameres are added to a chain consisting of

STRUCTURE AND CLASSIFICATION OF THE AttTHROPODA. 537

two or more somites by growth and gradual elaboration—what is called "budding"—of the anterior border of thehindermost somite. This hindermost somite is thereforedifferent from all the other somites, and is called the ' telson.'However long or short or heteromerised the chain may be,new metameres or somites are only produced at the anteriorborder of the telson, except in the Vertebrata." That is thegeneral law; but amongst some groups of metamerisedanimals partial exceptions to it occur. It is probably abso-lutely true for the Arthropoda from lowest to highest. It isnot so certain that it is true for the Chastopoda, and wouldneed modification in statement to meet the cases of fissi-parous multiplication occurring among Syllids and Naidids.In the Vertebrata, where tagmosis and heterosis of meromesand dislocation of meromes and tagmata are, so to speak,rampant, new formation of metameres (at any rate as repre-sented by important meromes) takes place at more than onepoint in the chain. Such points are found where two highlydiverse "tagmata" abut on one another. It is possible,though the evidence at present is entirely against the supposi-tion, that at such points in Arthropoda new somites may beformed.1 Such new somites are said to be "intercalated."The question of the intercalation of vertebi-Ee in the Verte-brata has received some attention. It must be rememberedthat a vertebra, even taken with its muscular, vascular, andueural accessories, is only a partial metamere—a merome, andthat, so far as complete inefcameres are concerned, the

1 The curious case of superabundant parapodia in the binder somites ofApus has already been cited and referred to as an example of autorbythmicmultiplication of meromes. There is some reason for regarding the extrapairs of legs as being "intercalated" after the formation of the somite as asingle unit or merome by growth from the telson. Supposing, as appears tobe the case, that as the Apus increases in size, the number of extra pairs oflegs on a non-terminal somite increases, these added meromes are certainlyintercalated, and represent incomplete intercalated metameres. The intercala-tion of new elements does not really go much further than this in Vertebrata,for a vertebra with its myoskcletal tissues is only a merome, and not a completemetamere.

VOL. 47, PART 4. NEW SEEIES. M M

538 E. RAY LANKESTT5B.

Vertebrata do conform to the same law as the Arthropods.Intercalation of meromes—branchial, vertebral, and dermal(fin-supports)—seems to have taken place in Vertebrata inthe fishes, while in higher groups intercalation of vertebrae inlarge series has been accepted as the only possible explana-tion of the structural facts established by the comparison ofallied groups. The elucidation of this matter forms a veryimportant part of the work lying to the hand of the investi-gator of vertebrate anatomy, and it is possible that theapplication of Groodrich's law (the seventh of our list) maythrow new light on the matter.

In regard to the diminution in the number of somites in thecourse of the historical development of those various groupsof metamerised animals, which have undoubtedly sprungfrom ancestors with more numerous somites than they them-selves possess, it appears that we may formulate thefollowiuglaws as the tenth, eleventh, twelfth, and thirteenth laws ofmetamerism.

The TENTH LAW is that individual somites tend to atrophyand finally disappear as distinct structures, most readily atthe anterior and the posterior ends of the series constitutingan animal body. This is very generally exhibited in the headof Arthropoda, where, however, the operation of the law islargely modified by fusion (see below). With regard to theposterior end of the body, the atrophy of segments does not,as a rule, affect the telson itself so much as the somites infront of it and its power of producing new somites. Some-times, however, the telson is very minute and non-chitinised

The ELEVENTH LAW may be stated thus:—Any somite inthe series which is the anterior or posterior somite of a tagmamay become atrophied, reduced in size, or partially abortedby the suppression of some oE its meromes ; and finally, sucha somite may disappear and leave no obvious trace in theadult structure of its presence in ancestral forms. This iscalled the excalation of a somite. Frequently, however, such"escalated" somites are obvious in the embryo or leave some


merome (e. g. neuromere, muscle, or chitin-plate) which canbe detected by minute observation (microscopic) as evidenceof their former existence. The somite of the masillipede(thirdpost-oral appendage) of Apus cancr i f ormis is agoodexample of a somite on its way to excalation. The third pra>oral and the prsemaxillary somites of Hexapod insects areinstances where the only traces of the vanished somite arefurnished by the microscopic study of early embryos. Theprsegenital somite of the Arachnida is an example of a somitewhich is preserved in some members of the group and par-tially or entirely excalated in other cases, sometimes withfusion of its remnants to neighbouring somites.

The TWELFTH LAW of metamerism might very well beplaced in logical order as the first. It is the law of l ipomer-isni, and asserts that just as the metameric condition isproduced by a change in the bodies of the descendants ofunisegmental ancestors, so highly metamerised forms, i. e.strongly segmented forms with specialised regions of differ-entiated metameres, may gradually lose their metamerisedstructure and become apparently and practically unisegmentalanimals. The change here contemplated is not the atrophyof terminal segments one by one so as to reduce the size ofthe animal and leave it finally as a single somite. On thecontrary, no loss of size or of high organisation is necessary.But one by one, and gradually, the metameric grouping ofthe bodily structures disappears. The cuticle ceases to bethickened in rings; the muscles of the body-wall overruntheir somite boundaries. Internal septa disappear. Thenerve-ganglia concentrate or else become diffused equallyalong the cords; one pair of renal coelomoducts and one pairof genital coelomoducts grow to large size and remain—therest disappear. The appendages atrophy or become limitedto one or two pairs, which are widely dislocated from theirancestral position. The animal ceases to present any indica-tion of metameric repetition of parts in its entire structure.Degrees in this process are frequently to be recognised. Wecertainly can observe such a change in the posterior region

540 B. BAY LANKBSTEB.

of some Arthropods, such as the hermit crabs and the spiders.Admitting that the Bchiurids are descended from Chfetopoda,such a change has taken place in them amounting to littleshort of complete lipomerism, though not absolutely complete.

Receut suggestions as to the origin of the Mollusca involvethe supposition that such an effacement of once well-markedmetamerism has occurred in them, leaving its traces only ina few structures such as the multiple gill-plumes and shell-shields of the Chitons and the duplicated renal sacs ofNautilus.

A further matter of importance in this connection is thatwhen the old metameres have been effaced a new secondarysegmentation may arise, as in the jointed worm-like body ofthe degenerate Acarus, Demodex f o l l icu lorum.

Such secondary annulation of the soft body calls to mindthe secondary annulation of the metameres of leeches andsome earthworms. Space does not permit of more than anallusion to this subject, but it is worth while noting thatthe secondary annuli marking the somites of leeches andLumbricidte in definite number and character are perhapscomparable to the redundant pairs of appendages on thehinder somites of Apus, and are in both cases examples ofindependent repetition of tegumentary meromes—a sort ofineffectual attempt to subdivide the somite which only pre-vails on the more readily susceptible meromes of the integu-ment.

The development of secondary metameric annulationswithin the area of a complete somite is not recorded amongArthropoda. It deserves distinct recognition as " hypo-metamerism " or formation of " sornatidia."

The last law of metamerism which we shall attempt toformulate here, as the THIRTEENTH, relates to the fusion orblending of neighbouring somites. There are, without doubt,a large number of important generalisations to be arrived athereafter from the further study of the metamerism ofVertebrata and the peculiar phenomena exhibited by thedislocated meromes of the vertebrate's somites. But this is


not the place in which to attempt an outline of the speciallaws of vertebrate metamerism. Fusion of adjacent somiteshas often been erroneously interpreted in the study ofArthropoda. There are, in fact, very varying degrees offusion which need to be carefully distinguished. The fol-lowing generalisation may be formulated :—" The homologousmeromes of two or more adjacent somites tend to fuse withone another by a blending of their substance. Very gene-rally, but not invariably, the fused meromes are found asdistinct separated structures in the embryo of the animalin which they unite at a later stage of growth." The .fusionof neighbouring meromes is often preceded by more or lessextensive atrophy of the somites concerned, and by a.rrest ofdevelopment in the individual ontogeny._, Thus a case offusion of partially atrophied somites may simulate the ap-pearance of incipient merogeuesis or formation of newsomites; and vice versa, incipient merogenesis may bemisinterpreted as a case of fusion of once separate andfully formed somites. Moreover the two phenomena, mero-genesis and fusion of meromes, actually occur side by sidein some cases, as in the pygidial shields of the Trilobitae andLimulus.

The most commonly noted cases of fusion of metameresare simply cases of the fusion of the tegumentary meromes—usually the terga only. Such a fusion has really no veryserious morphological importance: it is superficial andreadily acquired. It amounts to no more than the dispositionof chitinous cuticle of equal thickness over the area of theterga of the somites concerned instead of the thinning of thecuticular deposit at the adjacent borders of the somites.The somites consequently lose their hinge; they can nolonger be flexed one on the other. Atrophy of the musclesrelated to such flexure necessarily follows. The mesosomaticportion of the posterior carapace of Limulus is no more thansuch a superficial fusion: the other meromes of the anky-losed somites (appendages, neuromeres, blood-vessels, etc.)are unaffected. Such, too, is the case with the pygidial

542 E. BAY LANKESTEB.

shields of many Trilobites. On the other hand, the telson,which is joined in both these cases with the superficiallyfused segments by a fusion of its chitinous cuticle with thatof its last-formed or budded somite, can only take part inthe fusion as a result of arrest in its activity, which amountsto a late supervening atrophy. This arrest of the telson'sspecial bud growth may take place very early, in which casewe get a large telsonic shield and only a very few somites infront of it—none soldered to the telson as in Agnostus andIlenus; or it may take place later when eight post-cephalic(opisthosomatic) somites have been formed as in Limulus—the last two incompletely. Or, again, thirty or more somitesmay have been produced before the arrest takes place, andfifteen of these may be ankylosed with the telson to foi'm thepygidial shield (Phacops, etc.).

A more complete fusion of somites is that seen in the headof Arthropoda. The head or prosoma of Arthropod a is atagma consisting of one, two, or three prosthomeres orsomites in front of the mouth, and of one, two, three, up tofive or six opisthomeres. The cephalic tagma or prosomamay thus be more or less sharply divided into two sub-tagmata, the praa-oral and the post-oral.

The shifting of the mouth backwards in Arthropoda so asto allow segments which once were post-oral to take up aprseoral position, as prosthomeres, must be regarded as acase of dislocation of the meromes concerned (sixth law),like the forward travelling of a fish's pelvic fins. The anusdoes not appear to be liable to such dislocation in Arthro-poda, but it certainly does travel away from its parentalmetamere in the Vertebrata, and may possibly do so inCha;topoda when what must be called " lipomerism" orgeneral obliteration of a metameric ordering of parts sets in.Such "lipomerism" must be supposed to have affected theChastopod ancestors of the Sipunculids, if those latter wormsare to be traced genetically to the former, and the anus hasshifted to the anterior third of the body. However that maybe, the conception (first put forward by Lankester in 1875)

STfeUCTUBE ANO CLASSIFICATION OF l'HE AETHEOPODA. 543

(2) of the backward movement of the mouth in Arthropodafrom the first somite to the second, third, or even fourth inthe original post-oral series, is not only justified by embryo-logical observation of the shifting in question, but findsits parallel in other instances of the law of dislocation ofnieromes.

The fusion of the cephalic or prosoruatic somites not onlyextends to tegumentary structures, but to muscles, blood-vessels, and markedly to neuromeres. However, in theembryo of many Arthropoda the original neuromeres of thepraeoral somites can be distinguished, and in many casesthe coslomic cavities. Also it is a noteworthy fact that thetegumentary fusion (cephalic carapace, prosomatic carapace)appears sometimes to break down secondarily (e. g. Squillaamong Crustacea and Galeodes and Tarassidae among Arach-nida). It appears that we must recognise as a principlethat such fusions as the carapaces of Arthropoda can revertto the condition of free movable plates; and therefore wemust not assume that forms with fused tergal plates arenecessarily later, genetically, than allied forms with freemovable tergal plates.

When such reversion to a movable series of dorsal platesoccurs it must not be assumed that any coiTesponding changetakes place in the deeper meromes. On the whole, fusionand ankylosis of somites is not in itself necessarily a deep-seated or far-reaching process. It may or may not beaccompanied by dislocation • of important meromes or bylipomerism; whilst,—as for instance in the opisthosoma of thespiders, opiliones, and acari—dislocation and lipomerism mayoccur without fusion of tegumentary plates, and with, onthe contrary, a dwindling and eventual atrophy of suchplates.

The genei-al considerations as to metamerism set forthabove will enable us to proceed to a consideration of thecharacters which distinguish the various groups of Arthro-poda, and to justify the classification with which we started.

The Theory of the A r t h r o p o d Head.—The arthropod

544 E. BAY LANKESTER.

head is a tagma or group of somites which differ in numberand in their relative position in regard to the mouth, indifferent classes. In a simple Ohastopod (fig. 1) the headconsists of the first somite only; that somite is perforated bythe mouth, and is provided with a prostomium or praeorallobe. The prostomium is essentially a part or outgrowth ofthe first sornite, and cannot be regarded as itself a somite.It gives rise to a nerve-ganglion mass, the prostomial ganglion.In the marine Chastopods (the Polychasta) (fig. 2) we find

FIG. 1. FIG. 2.

Pr-

]?IG. 1.—Diagram of the Lead and adjacent region of an Oligo-chajte Clisetopod. Pr, tlie prostomium; «t, the mouth; A, theprostomial ganglion-mass or archicerebrum; I, II, III, ccelom ofthe first, second, and third somites. (From Goodrich, 'Q. J. Micr.Sci.,' vol. xi, p. 247.)

FIG. 2.—Diagram of the head and adjacent region of a PolychseteChsetopod. Letters as in Pig. 1, with the addition of T, prosto-mial tentacle ; Pa, parapodium. (From Goodrich.)

the same essential structure, but the prostomium may giverise to two or more tactile tentacles, and to the vesiculareyes. The somites have well-marked parapodia, and thesecond and third, as well as the first, may give rise totentacles which are directed forward, and thus contribute toform " the head." But the mouth remains as an inpushingof the wall of the first somite.

The Arthropoda are all distinguished from the Chastopodaby the fact that the head consists of one or more somiteswhich He in front of the mouth (now called prosthomeres),


as well as of one or more somites behind it (opisthomeres).The first of the post-oral somites invariably has its parapodiamodified so as to form a pair of hemignattis (mandibles).Twenty-five years ago the question arose as to whether thesomites in front of the mouth are to be considered as derivedfrom the prostomium of a Ohsetopod-like ancestor. Milne-Edwards and Huxley had satisfied themselves with discussingand establishing, according to the data at their command, thenumber of somites in the Arthropod head, but had not con-sidered the question of the n a t u r e of the praoral somites.Lankester (2) was the first to suggest that (as is actuallythe fact in the Nauplius larva of Crustacea) the prseoralsomites or prosthomeres and their appendages were ances-trally post-oral, but have become prsBoral " by adaptationalshifting of the oral aperture." This has proved to be a soundhypothesis, and is now accepted as the basis upon which theArthropod head must be interpreted (see Korschelt andHeider [3]). Further, the morphologists of the •'fiftiesappear, with few exceptions, to have accepted a preliminaryscheme with regard to the Arthropod head and Arthropodsegmentation generally, which was misleading aud causedthem to adopt forced conclusions and interpretations. I twas conceived by Huxley, among others, that the samenumber of cephalic somites would be found to be character-istic of all the diverse classes of Arthropoda, and that thesomites not only of the head, but of the various regions ofthe body, could be closely compared in their numericalsequence in classes so distinct as the Hexapods, Crustaceans,and Arachnids.

The view which it now appears necessary to take is, on thecontrary, this—viz. that all the Arthropoda are to be tracedto a common ancestor resembling a Chastopod worm, butdiffering from it in having lost its chastaa and in having aprosthomere in front of the mouth (instead of prostomiumonly) and a pair of hemignaths (mandibles) on the parapodiaof the buccal somite. From this ancestor Arthropods withheads of varying degrees of complexity have been developed

546 a. RAY LANKESTM.

characteristic of the different classes, whilst the parapodiaand somites of the body have become variously modified andgrouped in these different classes. The resemblances whichthe members of one class often present to the members ofanother class in regard to the form of the limb-branches(rami) of the parapodia, and the formation of tagmata(regions) are not hastily to be ascribed to common inherit-ance, but we must consider whether they are not due tohomoplasy—that is, to the moulding of natural selectionacting in the different classes upon fairly similar elementsunder like exigencies.

The structure of the head in Arthropods presents t h r eeprofoundly separated grades of structure dependent uponthe number of prosthomeres which have been assimilated bythe prteoral region. The classes presenting these distinctplans of head-structure cannot be closely associated in anyscheme of classification professing to be natural. Peripatus,the type genus of the class Onycliophora, stands at the baseof the series with only a single prosthomere (fig. 8). InPeripatus the prostoinium of the Chtetopod-like ancestor isatrophied, but it is possible that two processes on the frontof the head (FP) represent in the embryo the dwindledprostornial tentacles. The single prosthomere carries theretractile tentacles as its "parapodia." The second somiteis the buccal somite (II, fig. 3); its parapodia have hornyjaws on their ends, like the claws on the following legs(fig. 8), and act as hemignaths (mandibles). The study ofsections of the embryo establishes these facts beyond doubt.It also shows us that the neuromeres, no less than theembryonic coelomic cavities, point to the existence of one,and only one, prosthomere in Peripatus, of which the" Protocerebrum," P, is the neuromere, whilst the Deutero-cerebrum, D, is the neuromere of the second or buccalsomite. A brief indication of these facts is given by sayingthat the Onycliophora are " deuterognathous,"—that is tosay, that the buccal somite carrying the mandibular hemi-gnaths is the second of the whole series.

STRUCTURE AND CLASSIFICATION 01' THE ARTHROPODA. 647

What has become of the nerve-ganglion of the prostoiniallobe of the Chtetopod in Peripatus is not clearly ascertained,nor is its fate indicated by the study of the embryonic headof other Arthropods so far. Probably it is fused with theprotocerebruin, and may also be concerned in the history ofthe vei'y peculiar paired eyes of Peripatus, which are likethose of Chastopods in structure—viz. vesicles with an intra-

FlG. 3.—Diagram of the liead and adjacent region of Peripatus.Monoprostliomerous. in, Mouth; I, coelom of the first somitewhich carries the antennae, and is in front of the mouth; II, coelomof the second somite, which carries the mandibles (hence deutero-gnathous); III and IV, coelom of the third and fourth somites;F.P., rudimentary frontal processes, perhaps representing the pro-stomial tentacles of Poljchseta; Ant, antenna or tactile tentacle; Md,mandible; Op, oral papilla; P, prot.oceiebrum or foremost cerebralmass belonging to the first somite; D, deuterocerebrum, consistingof ganglion cells belonging to the second or mandibular somite.(After Goodrich.)

vesicular lens, whereas the eyes of all other Arthropods haveessentially another structure, being "cups" of the epidermis,in which a knob-like or rod-like thickening of the cuticle isfitted as refractive medium.

In Diplopoda (Julus, etc.) the results of enibryologicalstudy point to a composition of the front part of the headexactly similar to that which we find in Onychophora. Theyare deuterognathous.

The Arachnida present the first stage of progress. Hereembryology shows that there are two prosthomeres (fig. 4),

548 B. KAY LANKESTEE.

and that the guatkobases of the chelas which act as the firstpair of heinignaths, are carried by the third somite. TheAraclmida are therefore tritoguathous. The two prostho-meres are indicated by their cceloinic cavities in the embryo(I and II, fig. 4), and by two neuromeres, the protocerebrumand the deuterocerebrum. The appendages of the firstprosthomere are not present as tentacles, as in Peripatus andDiplopods, but are possibly represented by the eyes or possiblyaltogether aborted. The appendages of the second prostho-mere are the well-known chelicerte of the Arachnids, rarely,

FIG. 4.—Diagram of the head and adjacent region of an Arachnid.Diprostbomerous in the adult condition, though embryologically theappendages of somite II and the somite itself are, as here drawn,not actually in front of the mouth. E, lateral eye ; Ch, chelicera ;HI, mouth; P, piotocerebrum ; D, deuterocerebium ; I, II, III, IV,ccelom of the first, second, third, and fourth somites. (AfterGoodrich.)

if ever, antenniform, but modified as " retroverts" or clasp-knife fangs iu spiders.

The Crustacea (fig. 5) and the Hexapoda (fig. 6) agree inhaving three somites in front of the mouth, and it is probable,though not ascertained, that the Chilopoda (Scolopendra, etc.)are in the same case. The three prosthomeres or prteoralsomites of Crustacea due to the sinking back of the, mouthone somite farther than in Araclmida are not clearly indicatedby ccelomic cavities in the embryo, but their existence isclearly established by the development and position of theappendages and by the neuromeres.

STRUCTURE AND CLASSIFICATION OF THE ARTHBOPODA. 549

The eyes in some Crustacea are mounted on articulatedstalks, and from the fact that they can after injury bereplaced by antenna-like appendages it is inferred that theyrepresent the parapodia of the most anterior prosthomere.The second prosthomere carries the first pair of antennas and

FIG. 5. FIG. 6.

TIG. 5.—Diagram of the head of a crustacean. Triproslho-nierous. F. P, frontal processes (observed in Cinhiped nauplius-larvae), probably representing the prostomifil tentacles of Chaetopods;e, eye ; Ant1, first pair of antennae ; Ant2, second pair of antennae;md, mandidle; ma;1, imp, first and second pairs of maxillse; m,mouth; I, II, III, tlie three prosUiomeres; IV, V, VI, the threesomites following the mouth; P, proctocerebrum ; D, deutero-cerebrum; T, tritocerebrum. (After Goodrich.)

FIG. 6.—Diagram of the head of a Hexapod insect, e, eye; ant,antenna; md, mandible; ma;1, first maxilla; ma2, second maxilla;in, mouth; I, region of the first or eye-bearing piosthomere; II,coelom of the second antenna-bearing prosthomere; III, coelom ofthe third prosthomere devoid of appendages; IV, V, and VI, coelomof the fourth, fifth, and sixth somites ; P, protocerebrum belongingto the first, prosthomere ; D, deutei'Ocerebrum belonging to thesecond prosthomere; T, tritocerebrum belonging to the third pros-thomere. (After Goodrich.)

the third the second pair of antennae. Sometimes this pairof appendages has not a merely tactile jointed ramus, but isconverted into a claw or clasper. Three neuromeres—aproto-, deutero-, and tritocerebrum—corresponding to thosethree prosthomeres, are sharply marked in the embryo. Thefourth somite is that in which the mouth now opens, andwhich accordingly has its appendages converted into

550 E. KAY LANKESTEE.

hemignafchous mandibles. The Crustacea are tetartogna-thous.

The history of the development of the head has been care-fully worked out in the Hexapod insects. As in Crustaceaand Arachnida, a first prosthomere is indicated by the pairedeyes and the protocerebrum; the second prosthomere has awell-marked coelomic cavity, carries the antennae, and has thedeuterocerebrurn for its neuroniere. The third prosthomereis represented by a well-marked pair of coelomic cavities andthe tritocerebrum (III, fig. 6), but has no appendages. Theyappear to have aborted. The existence of this third prostho-mere, corresponding to the third prosthomere of the Crustacea,is a strong argument for the derivation of the Hexapoda, andwith them the Chilopoda, from some offshoot of the Crustaceanstem or class. The buccal somite, with its mandibles, is inHexapoda, as in Crustacea, the fourth: they are tetarto-gnathous.

The adhesion of a greater or less number of somites to thebuccal somite posteriorly (opisfchomeres) is a matter of im-portance, but of minor importance, in the theory and historyof the Arthropod head. In Peripatus no such adhesion orfusion occurs. In Diplopoda two opisthomeres—that is tosay, one in addition to the buccal somite—are united by afusion of their terga with the terga of the prosthomeres.Their appendages are respectively the mandibles and thegnathochilar i a m.

In Arachnida the highest forms exhibit a fusion of thetergites of five post-oral somites to form one continuous cara-pace united with the terga of the two prosthomeres. Thefive pairs of appendages of the post-oral somites of the heador prosoma thus constituted all primitively carry gnathobasicprojections on their coxal joints, which act as hemignaths; inthe more specialised forms the mandibular gnathobases ceaseto develop.

In Crustacea the fourth or mandibular somite never hasless than the two following somites associated with it by theadaptation of their appendages as jaws, and the ankylosis of

STRUCT UK E AND CLASSIFICATION OF THE ARTHROPODA. 551

their terga with that of the prosthomeres. But in higherCrustacea the cephalic "tagma" is extended, and moresomites are added to the fusion, and their appendages adaptedas jaws of a kind.

The Hexapoda are not known to us in their earlier or moreprimitive manifestations; we only know them as possessed ofa definite number of somites arranged in definite numbers inthree great tagmata. The head shows two jaw-bearingsomites besides the mandibnlar somite (V, VI, in fig. 6) —thus six in all (as in some Crustacea), including prosthomeres,all ankylosed by their terga to form a cephalic shield. Thereis, however, good embryological evidence in some Hexapodsof the existence of a seventh somite, the supra-lingual,occurring between the somite of the mandibles and the somiteof the first maxillas (4). This segment is indicated einbryo-logically by its paired coelomic cavities. It is practically anexcalated somite, having no existence in the adult. It is pro-bably not a mere coincidence that the Hexapod, with its tworudimentary somites devoid of appendages, is thus found topossess twenty-one somites, including that which carries theanus, and that this is also the number present in the Mala-costracous Crustacea.

The Segmental Lateral Appendages or Limbs ofArthropoda.—It has taken some time to obtain any generalacceptance of the view that the parapodia of the Cheetopodaand the limbs of Arthropoda are genetically identical struc-tures ; yet if we compare the parapodium of Tomopteris or ofPhyllodoce with one of the foliaceons limbs of Branchipusor Apus the correspondences of the two are striking. Anerroneous view of the fundamental morphology of the crus-tacean limb, and consequently of that of other Arthropoda,came into favour owing to the acceptance of the highlymodified limbs of Astacus as typical. Protopodite, endopodite,exopodite, and epipodite were considered to be the morpho-logical units of the crustacean limb. Lankester (5) hasshown (and his views have been accepted by ProfessorsKorschelt and Heider in their treatise on 'Embi'yology')

552 B. JUT LANKESTER.

that the limb of the lowest Crustacea, such as Apus, consistsof a corm or axis which may be jointed, and gives rise to out-

FIG. 7. FIG. 8.

nt.c.

PIG. 7.—Diagram of the somite-appendage or parapodiuni of aPolyclisete Clisetopod. The choetrc are omitted. Ax, the axis;nr. c, neuropodial cirrhus; nr. I1, nr. I1, neuropodial lobes or endites;nt. c, notopodial cirrhus ; nt. I1, nt. P, notopodial lobes or exites.The parapodium is represented with its neural or ventral surfaceuppermost. (Original.)

PIG. 8.—Three somite-appendages or parapodia of Peripatus.A, a walking leg ; p to ;;', the characteristic "pads;"/", the foot;el1, cP, the two claws; B, an oral papilla, one of the second pair ofpost-oral appendages; C, one of the first post-oral pair of appendagesor mandibles; cl\ cP, the greatly enlarged claws. (Compare A.)

The appendages are represented with the neural or ventralsurface uppermost. (Original.)

growths, either leaf-like or filiform, on its inner and outermargins (endites and exites). Such a corm (see figs. 9 and10), with its outgrowths, may be compared to the simple


parapodia of Chtetopoda with cirrhi and branchial lobe(fig. 7). It is by the specialisation of two "eudi tes" thabthe endopodite and exopodite of higher Crustacea are formed,whilst a flabelliform exite is the homogen or genetic equiva-lent of the epipodite (see Lankester, " Observations andReflections on Apus canc r i fo rmi s / ' fQ. J. Micr. Sci.').The reduction of the outgrowth-bearing " corm" of theparapodium of either a Chtetopod or an Arthropod to a simplecylindrical stump, devoid of outgrowths, is brought about

FIG. 9.—The second thoracic (fifth post-oral) appendage of theleft side of Apus cancriformis, placed with its ventral or neuralsurface uppermost to compare with Figs. 7 and 8. 1, 2, the twosegments of the axis; en\ t,he gnathobase ; en- to en6, the fivefollowing "endites;" fl, the flahellum or anterior exite; ir, thebract or"posterior exite. (After Lankester, ' Q. J. Micr. Sci.,' vol.xxi, 1881.)

when mechanical conditions favour such a shape. We see itin certain Chsetopods (e. g. Hesione) and in the ArthropodPeripatus (fig. 8). The conversion of the Arthropod's limbinto a jaw, as a rule, is effected by the development of anendite near its base into a hard, chitinised, and often toothedgnathobase (see figs. 9 and 10, en1). It is not true that allthe biting processes of the Arthropod limb are thus produced,—for instance, the jaws of Peripatus are formed by the axisor corm itself; whilst the poison-jaws of Chilopods, as alsotheir maxillas, appear to be formed rather by the apex orterminal region of the ramus of the limb; but the opposingjaws ( = hemignaths) of Crustacea, Arachnida, and Hexapodaare gnathobases, and not the axis or corm.. The endopodite(corresponding to the fifth endite of the limb of Apus, seefig. 9) becomes in Crustacea the " walking leg " of the mid-

VOL. 4 7 , PART 4.—NKW SEKTES. U N '

554 E. RAY LANKBSTKR.

region of the body ; it becomes the palp or jointed process ofanterior segments. A second ramus, the "exopodite," often isalso retained in the form of a palp or feeler. In Apus, as

FIG. 10.—The first thoracic (fourth post-oral) appendage ofApus cancriformis (right side). Axx to A-v*, the four seg-ments of the axis with muscular bands; En\ gnathobase ; En1 to En?,the elongated jointed endites (rami); En6, the rudimentary sixthendite (exopodite of higher Crustacea); Fl, the flabellum whichbecomes the epipodite of higher forms; Br, the bract devoid ofmuscles and respiratory in function. (AJter Laukester, 'Q. J. Micr.Sci.,' vol. xxi, 1881.)'

the figure shows, there are four of these " antenna-like"palps or filaments on the first thoracic limb. A commonmodification of the chief ramus of the Arthropod parapodiumis the chela or nipper formed by the elongation of the pen-

1 This figure lias been re-drawn for the present reprint.—E, R. L.

STEUCTTJRE AND CLASSIFICATION OP THIS AETHEOPODA. 555

ultimate joint of the ranius, so that the last joint works on it—as, for instance, in the lobster's claw. Such chelate ramior limb-branches are independently developed in Crustaceaand in Arachnida, and are carried by somites of the body

rvf

FIG. 11.—Diagram to show the derivation of the unit or " om-matidium " of the compound eye of Crustacea and Hexapoda, C,from a simple monomeniscous monostichous eye resembling thelateral eye of a scorpion, A, or the unit of the compound lateral eyeof Limulus (see article AUACHNIDA, Figs. 22 and 23). B repre-sents an intermediate hypothetical form in which the cells beneaththe lens are beginning to be superimposed as corneagen, vitrella,and retinula, instead of standing side by side in horizontal series.The black represents the cuticular product of the epidermal cells ofthe ocular area, taking the form either of lens, cl, of crystallinebody, cry, or of rhabdom, rhab; hy, hypodermis or epidermal cells;corn, laterally placed cells in the simpler stage A, which like thenerve-end cells, vil1 and ret1, are corneagens or lens-producing;corn, specialised corneagen or lens-producing cells ; vil1, potentialvitrella cells with cry, potential crystalline body now indistinguish-able from retinula cells and rhabdomeres ; vil, vitrella cell with cry,its contained cuticular product, the crystalline cone or body ; vil1,rhab1, retinula cells and rhabdom of scorpion undifferentiated fromadjacent cells, vil1; ret1, retinula cell; rhab, rhabdom ; nf, opticnerve-fibres. (Modified from Watasae.)

which do not correspond in position in the two groups. Therange of modification of which .the rami or limb-branches ofthe limbs of Arthropoda are capable is veiy large, and inallied orders, or even families or genera, we often find whatis certainly the palp of the same appendage (as determined

556 H. RAY LANKESTER.

by numerical position of the segments)—in one case antenni-form, in another chelate, in another pediform, and in anotherreduced to a mere stump or absent altogether. Veryprobably the power which the appendage of a given segmenthas of assuming the perfected form and proportions previouslyattained by the appendage of another segment must beclassed as an instance of " homoeosis," not only where such achange is obviously due to abnormal development or injury,but also where it constitutes a difference permanentlyestablished between allied orders or smaller groups, or betweenthe two sexes.

The most extreme disguise assumed by the Arthropodparapodium or appendage is that of becoming a mere stalksupporting an eye, a fact which did not obtain generalcredence until the experiments of Herbst, in 1895, who found,on cutting off the eye-stalk of Patemon, that a jointedantenna-like appendage was regenerated in its place. Sincethe eye-stalks of Podothalmate Crustacea represent append-ages, we are forced to the conclusion that the sessile eyes ofother Crustacea, and of other Arthropoda generally, indicatethe position of appendages which have atrophied.1

From what has been said it is apparent that we cannot, inattempting to discover the affinities and divergences of thevarious forms of Arthropoda, attach a very high phylogeneticvalue to the coincidence or divergence in form of tbe append-ages belonging to the somites compared with one another.

The principal forms assumed by the Arthropod parapodiumand its rami may be thus enumerated :

(1) Axial corm well developed, unsegmented or with twoto four segments ; lateral endites and exites (vami) numerousand of various lengths (certain limbs of lower Crustacea).

(2) Corm, with short, unsegmented rami, forming a flat-1 H. Milne-Edivards, who was followed by Huxley, long ago formulated the

conclusion that the eye-stalks of Crustacea are modified appendages, basinghis argument on a specimen of Palinmus (figured in Bateson's book) (1), inwhich the eye-stalk of one side is replaced by an antenniform palp. Hofer (6)in 1894 described a similar case in Astacus.

STBUCTURU AND CLASSIFICATION OF THE AETHBOPODA. 557

tened foliaceous appendage, adapted to swimming and respira-tion (trunk limbs of Phyllopods).

(3) Corm alone developed, with no endites or exites, butprovided with terminal chitinous claws (ordinary leg ofPeripatus), with terminal jaw teeth (jaw of Peripatus), orwith blunt extremity (oral papilla of same) (see fig. 8).

(4) Three of the raini of the primitive limb (endites 5 and6 and exite 1) specially developed as endopodite, exopodite,and epipodibe, the first two often as firm and strongly chitin-ised, segmented, leg-like structures; the original axis or cormreduced to a basal piece, with or without a distinct gnatho-base (endite 1), typical trirarnose limb of higher Crustacea.

(5) One ramus (the endopodite) alone developed—theoriginal axis or corm serving as its basal joint with orwithout gnathobase. This is the usual uniraruose limbfound in the various classes of Arthropoda. It varies asto the presence or absence of the jaw process and as to thestoutness of the segments of the ramus, their number (fre-quently six,plus the basal corm),and the modification of thefree end. This may be filiform, or brush-like, or lamellatewhen it is an antenna or palp; a simple spike (walking legof Crustacea, of other aquatic forms, and of Chilopods andDiplopods); the terminal joint flattened (swimming leg ofCrustacea and Gigantostraca); the terminal joint providedwith two or with three recurved claws (walking leg of manyterrestrial forms—e.g. Hexapoda and Arachnida); thepenultimate joint with a process equal in length to thelast joint, so as to form a nipping organ (chelas of Crusta-ceans and Arachnids); the last joint reflected and movableon the penultimate, as the blade of a clasp-knife on itshandle (the retrovert, toothed so as to act as a biting jawin the Hexapod Mantis, the Crustacean Squilla, andothers; with the last joint produced into a needle-likestabbing process in spiders).

(6) Two rami developed (usually, but perhaps not always,the equivalents of the endopodite and exopodite) supportedon the somewhat elongated corm (basal segmeno). This is

558 B. RAY 1ANKBSTEB.

the typical "biramose limb" often found in Crustacea. Therami may be flattened for swimming, when it is " a biramoseswitnmeret," or both or only one may be filiform and finelyannulate; this is the form often presented by the antennasof Crustacea, and rarely by piaaoral appendages in otherArthropods.

(7) The endopoditic ramus is greatly enlarged and flat-tened, without or with only one jointing, the conn (basalsegment) is evanescent; often the plate-like endopodites of apair of such appendages unite in the middle line with oneanother or by the intermediary of a sternal upgrowth andform a single broad plate. (These are the plate-like switn-merets and opercula of Grigantostraca and Limulus amongArachnids and of Isopod crustaceans. They may have rudi-mentary exopodites, and may or may not have branchialfilaments or lamella) developed on their posterior faces. Thesimplest form to which they may be reduced is seen in thegenital operculum of the scorpion.)

(8) The gnathobase becomes greatly enlarged and notseparated by a joint from the corm; ib acts as a hemignathor half-jaw working against its fellow of the opposite side.The endopodite may be retained as a small segmented palpat the side of the gnathobase or disappear (mandible ofCrustacea, Chilopoda, and Hexapoda).

(9) The corm becomes the seat of a development of aspecial visual organ, the Arthropod eye (as opposed tothe Chfetopod eye). Its jointing (segmentation) may beretained, but its rami disappear (podophthalmous Crustacea).Usually it becomes atrophied, leaving the eye as a sessileorgan upon the prseoral region of the body. (The eye-stalkand sessile lateral eyes of Arthropoda generally, exclusive ofPeripatus.)

(10) The forms assumed by special modification of theelements of the parapodium iu the maxillee, labium, etc.,of Hexapods, Chilopods, Diplopods, and of various Crustaceadeserve special enumeration, but cannot be dealt with with-out ample space and illustration.

STRUCTURE AND CLASSIFICATION OF TfiE AETHBOPODA. 559

It may be pointed out that the most radical differencepresented in this list is that between appendages consistingof the conn alone without rarni (Onychophora) and thosewith more or less developed rami (the rest of the Arthropoda).In the latter class we should distinguish three phases: (a)those with numerous and comparatively undeveloped rarni;(b) those with three, or two highly developed rami, or withonly one—the corrn being reduced to the dimensions of amere basal segment; (c) those reduced to a secondarysimplicity (degeneration) by overwhelming development ofone segment (e. g. the isolated gnathobase often seen as"mandible" and the genital opercnlum).

There is no reason to suppose that any of the forms oflimb observed in Arthropoda may not have been indepen-dently developed in two or more separate diverging lines ofdescent.

Branchiae.—In connection with the discussion of the limbsof Arthropods a few words should be devoted to the gill-processes. It seems probable that there are branchial plumesor filaments in some Arthropoda (some Crustacea) which canbe identified with the distinct branchial organs of Chastopoda,which lie dorsad of the parapodia and are not part of theparapodium. On the other hand, we cannot refuse to admitthat any of the processes of an Arthropod parapodium maybecome modified as branchial organs, and that, as a rule,branchial outgrowths are easily developed, de novo, in alJthe higher groups of animals. Therefore it seems to be,with our present knowledge, a hopeless task to analyse thebranchial organs of Arthropoda and to identify them genetic-ally in groups.

A brief notice must suffice of the structure and history ofthe Byes, the Tracheas, and the so-called Malp igh iantubes of Arthropoda, though special importance attachesto each in regard to the determination of the affinities of thevarious animals included in this great sub-phylum.

The Eyes.—The Arthropod eye appears to be an organ ofspecial character developed in the common ancestor of the

560 B. EAT LANKESTER.

Euarthropoda, and distinct from the Chastopod eye, which isfound only in the Onychophora where the true Arthropodeye is absent. The essential difference between these twokinds of eye appears to be that the Chaatopod eye (in itshigher developments) is a vesicle enclosing the lens, whereasthe Arthropod eye is a pit or series of pits into which theheavy chitinous cuticle dips and enlarges knobwise as a lens.Two distinct forms of the Arthropod eye are observed—themonomeniscous (simple) and the polymeniscous (compound).The nerve-end cells, which lie below the lens, are part of thegeneral epidermis. They show in the monomeniscous eye(see article ARACHNIDA,1 fig. 26) a tendency to- group them-selves into " retinulse," consisting of five to twelve cellsunited by vertical deposits of chitin (rhabdoms). In thecase of the polymeniscous eye (fig. 23, article ARACHNIDA) asingle retinula or group of nerve-end cells is grouped beneatheach associated lens. A further complication occurs in eachof these two classes of eye. The monomeniscous eye is rarelyprovided with a single layer of cells beneath its lens; whenit is so, it is called monostichous (simple lateral eye ofscorpion, fig. 22, article ARACHNIDA). More usually, by aninfolding of the layer of cells in development, we get threelayers under the lens; the front layer is the corneagen layer,and is separated by a membrane from the other two, whichmore or less fuse and contain the nerve-end cells (retinallayer). These eyes are called diplostichous, aud occur inArachnida aud Hexapoda (fig. 24, in article ARACHNIDA.).

On the other hand, the polymeniscous eye undergoesspecial elaboration on its lines. The retinulas become elon-gated as deep and very narrow pits (fig. 11 and explanation),and develop additional cells near the mouth of the narrowpit. Those nearest to the lens are the corneagen cells of thismore elaborated eye, and those between the original retinulacells and the corneagen cells become firm and. transparent.They are the crystalline cells or vitrella (see Watase, 7).

1 Tuis article will be reproduced from the ' Encyclopedia' in the nextnumber of this Journal,—E. It. L.


Each such complex of cells underlying the lenticle of acompound eye is called an " ommatidium;" the entire massof cells underlying a monomeniscous eye is an "ommataaum."The ommatasum, as already stated, tends to segregate intoretinulse which correspond potentially each to an ommatidiumof the compound eye. The ommatidium is from the firstsegregate, and consists of few cells. The compound eye ofthe king-crab (Limulus) is the only recognised instance ofommatidia in their simplest state. Bach can be readily com-pared with the single-layered lateral eye of the scorpion. InCrustacea and Hexapoda of all grades we find compoundeyes with the more complicated ommatidia described above-We do not find them in any Arachnida.

It is difficult, in the absence of more detailed knowledge asto the eyes of Chilopoda and Diplopoda, to give full value tothese facts in tracing the affinities of the various classes ofArthropods. But they seem to point to a community oforigin of Hexapods and Crustacea in regard to the com-plicated ommatidia of the compound eye, and to a certainisolation of the Arachnida, which are, however, traceable, sofar as the eyes are concerned, to a distant common originwith Crustacea and Hexapoda through the very simple com-pound eyes (monostichous, polymeuiscous) of Limulus.

The Trachete.—In regard to tracheae the very naturaltendency of zoologists has been until lately to consider themas having once developed and once only, and therefore to holdthat a group " Tracheata " should be recognised, includingall tracheate Arthropods. We are driven by the conclusionsarrived at as to the derivation of the Arachnida from branchi-ate ancestors, independently of the other tracheate Arthropods(see article ARACHNIDA), to formulate the conclusion thattrachete have been independently developed in the Arachnidanclass. We are also, by the isolation of Peripatus and theimpossibility of tracing to it all other tracheate Arthropoda,or of regarding it as a degenerate offset from some one of thetracheate classes, forced to the conclusion that the tracheae ofthe Onychophora have been independently acquired. Having

562 E. RAY LANKESTEft.

accepted these two conclusions, we formulate the generalisa-tion that tracheae can be independently acquired by variousbranches of Arthropod descent in adaptation to a terrestrialas opposed to an aquatic mode of life. A great point ofinterest, therefore, exists in the knowledge of the structureand embryology of tracheae in the different groups. It mustbe confessed that we have not such full knowledge on thishead as could be wished for. Tracheae are essentially tubeslike blood-vessels—apparently formed from the same tissueelements as blood-vessels—which contain air in place of blood,and usually communicate by definite orifices, the trachea!stigmata, with the atmosphere. They are lined internally bya cuticular deposit of chitin. In Peripatus and the Diplopodsthey consist of bunches of fine tubes which do not branch,but diverge from one another; thechitinouslining is smooth.In the Hexapods and Ohilopods, and the Arachnids (usually),they form tree-like branching structures, and their finestbranches are finer than any blood capillary, actually in somecases penetrating a single cell and supplying it with gaseousoxygen. In these forms the chitinous lining of the tubes isthickened by a close-set spiral ridge similar to the spiralthickening of the cellulose wall of the spiral vessels of plants.It is a noteworthy fact that other tubes in these same terres-trial Arthropoda—namely, the ducts of glands—are similarlystrengthened by a chitinous cuticle, and that a spiral orannular thickening of the cuticle is developed in them also.Ohitin is not exclusively an ectodermal product, but occursalso in cartilaginous skeletal plates of mesoblastic origin(connective tissue). The immediate cavities or pits into whichthe tracheal stigmata open appear to be in many cases ecto-dermic in sinkings, but there seems to be no reason (based onembryological observation) for regarding the tracheae as aningrowth of the ectoderm. They appeal', in fact, to be anair-holding modification of the vasifactive connective tissue.Tracheae are abundant just in proportion as blood-vesselsbecome suppressed. They are reciprocally exclusive. Itseems not improbable that they are two modifications of the

STBUCTUBE AND CLASSIFICATION OF THE ARTHROPODA. 563

same tissue elements. In Peripatus the stigmatic pits atwhich the tracheae communicate with the atmosphere arescattered and not definite in their position. In other casesthe stigmata are definitely paired and placed in a few segmentsor in several. It seems that we have to suppose that thevasifactive tissue of Arthropoda can readily take the form ofair-holding instead of blood-holding tubes, and that thissomewhat startling change in its character has taken placeindependently in several instances—viz. in the Onychophora,in more than one group of Arachuida, in Diplopoda, and,again, in the Hexapoda and Chilopoda.

The Malp igh ian Tubes.—This name is applied to thenumerous fine csecal tubes of noticeable length developed fromthe proctodasal invert of ectodermal origin in Hexapods.These tubes are shown to excrete nitrogenous waste productssimilar to uric acid. Tubes of renal excretory function in alike position occur in most terrestrial Arthropoda—viz. inChilopoda, Diplopoda, and Arachnida. They are also foundin some of the semi-terrestrial and purely aquatic AmphipodCrustaceans. But the conclusion that all such tubes areidentical in essential character seems to be without founda-tion. The Malpighian tubes of Hexapods are outgrowths ofthe proctodteum, but those of scorpion and the AmphipodCrustacea are part of the meteuteron or endodermal gut,though originating near its junction with the proctodseum.Hence the presence or absence of such tubes cannot be usedas an argument as to affinity without some discrimination.The scorpion's so-called Malpighian tubes are not the sameorgans as those so named in the other Tracheata. Such i-enalcaacal tubes seem to be readily evolved from either mefcenteronor proctodseum when the conditions of the outwash of nitro-genous waste products are changed by the transference fromaquatic to terrestrial life. The absence of such renal casca inLimulus and their presence in the terrestrial Arachnida isprecisely on a parallel with their absence in aquatic Crustaceaand their presence in the feebly branchiate Amphipoda.

We shall now pass the groups of the Arthropoda in review,

564 E. RAY LANKBSTEK.

attempting to characterise them in such away as will indicatetheir probable affinities aud genetic history.

SUB-PHYLUM ARTHROPODA.—The characters of the sub-phylum, and those of the associated, sub-phyla Cheetopoda andRotifera, have been given above, as well as the generalcharacters of the phylum Appendiculata which comprisesthese great sub-phyla.

Grade A.—Hyparthropoda.

Hypothetical forms.Grade B.—Protarthropoda.

(a) The integument is covered, by a delicate soft cuticle(not firm or plated) which allows the body and. its appendagesgreat range of extension and contraction.

(b) The paired claws on the ends of the parapodia and thefang-like modifications of these on the first post-oral append-dages (mandibles) are the only hard chitinous portions of theintegument.

(c) The head is deuterognathous,—that is to say, there isonly one prosthomere, and accordingly the first and only pairof hemignaths is developed by adaptation of the appendagesof the second somite.

(d) The appendages of the third somite (second post-oral)are clawless oral papillse.

(e) The rest of the somites carry equi-formal simple append-ages, consisting of a coim or axis tipped with two chitinousclaws and devoid of rami.

(/) The segmentation of the body is anomeristic, therebeing no fixed number of somites characterising all the formsincluded.

(g) The pair of eyes situated on the prosthomere are not ofthe Euarthropod type, but resemble those of Chsetopods (henceNereid- ophthalmous).

(h) The muscles of the body-wall and gut do not consist oftransversely striped muscular fibre, but of the unstriped tissueobserved also in Chastopoda.

(i) A pair of ccelomoducts is developed in every somite,


including the prosthomere, in which alone it atrophies in laterdevelopment.

(j) The ventral nerve-cords are widely separated,—in fact,lateral in position.

(k) There are no masses of nerve-cells forming a ganglion(neuromere) in each somite. (In this respect the Protarthro-poda are at a lower stage than most of the existing Chaeto-poda.)

(I) The genital ducts are formed by the enlargement of thecoalomoducts of the penultimate somite.

Class (Unica).—ONTCHOPHOBA.

With the characters of the grade : add the presence withinthe body oE fine unbranched trachea! tabes, devoid of spiralthickening, opening to the exterior by numerous irregularlyscattered trachea! pits.

Genera—Eoperipatus, Peripatopsis, Opisthopatus, etc.

Grade C (of the Arthropoda).—Euarthropoda.

(a) Integument heavily plated with firm chitinous cuticle,allowing no expansion and retraction of regions of the bodynor change of dimensions, except, in some cases, a dorso-ventral bellows movement. The separation of the heavierplates of chitin by grooves of delicate cuticle results in thehinging or jointing of the body and its appendages, and theconsequent flexing and extending of the jointed pieces.

(b) Claws and fangs are developed on the branches or ramiof the parapodia, not on the end of the axis or corm.

(c) The head is either deuterognathous, tritognathous, ortetartognathous.

(d) Rarely only one, and usually at least two, of thesomites following the mandibular somite carry appendagesmodified as jaws (with exceptions of a secondary origin).

(e) The rest of the somites may all carry appendages,or only a limited number may carry appendages. In allcases the appendages primarily develop rami or branches

566 E. RAY LANKESTEB.

which form the limbs, the primitive axis or corm beingreduced and or' insignificant size. In the most primitivestock all the post-oral appendages had gnathobasic outgrowths.

(/) The segmentation of the body is anomomeristic in themore archaic members of each class, nomomeristic in thehigher members.

(g) The two eyes of Chsetopod structure have disappeared,and are replaced by the Euarthropod eyes.

(h) The muscles in all parts of the body consist of stripedmuscular fibre, never of unstriped muscular tissue.

(i) The coelomoducts are suppressed in most somites, andretained only as the single pair of genital ducts (very rarelymore numerous), and in some also as the excretory glands(one or two pairs).

(j) The ventral nerve-cords approach one another in themid-ventral line behind the mouth.

(k) The nerve cells of the ventral nerve-cords are segregatedas paired ganglia in each somite, often united by meristicdislocation into composite ganglia.

(1) The genital ducts may be the ccelomoducts of the pen-ultimate or antepenultimate or adjacent somite, or of asomite placed near the middle of the series, or of a somitefar forward in the series.

Class 1 (of the Euarthropoda).—DIPLOPODA.

The head has but one prosthomere (monoprosthomerous),and is accordingly deuterognathons. This carries short-jointedantennas (in one case biramose) and eyes, the structure anddevelopment of which require further elucidation. Only onesomite following the first post-oral or mandibular segmenthas its appendages modified as jaws.

The somites of the body, except in Pauropus, either fuseafter early development and form double somites with twopairs of appendages (Julus, etc.) or present legless and leg-bearing somites alternating.


Somites, anomomeristic, from 12 to 150 in the post-cephalicseries.

The genital ducts open in the fourth, or between the fourthand fifth post-oral somite.

Terrestrial forms with small-jointed legs formed by adapta-tion of a single ramus of the appendage. Tracheas arepresent.

• Note.—The Diplopoda include the Juliformia, the Sym-phyla (Scolopendrella), and Pauropoda (Pauropus). Theywere until recently classified with the Chilopoda (centipedes),with which they have no close affinity, but only a superficialresemblance. (Compare the definition of the class Chilopoda.)

The movement of the legs in Diplopoda is like that of thoseof Peripatus, of the Phyllopod Crustacea, and of the para-podia of Chsetopoda, symmetrical and identical on the twosides of the body. The legs of Chilopoda move in alternatinggroups on the two sides of the body ; this implies a verymuch higher development of nerves and muscles in that group.1

Class 2 (of the Euarthropoda).—ARACHNIDA.

Head tritognathous and diprosthomerous,—that is to say,with two prosthomeres; the first bearing typical eyes, thesecond a pair of appendages reduced to a single ramus, whichis in more primitive forms antenniform, in higher formschelate or retrovert. The ancestral stock was pantognatho-basic, i. e. had a gnathobase or jaw process on every para-podium. As many as six pairs of appendages following themouth may have an enlarged gnathobase actually functionalas a jaw or hemignath, but a ramus is well developed on eachof these appendages either as a simple walking leg, a palp,or a chela. In the more primitive forms the appendage ofevery post-oral somite has a gnathobase and two rami; inhigher specialised forms the guathobases maybe atrophied inevery appendage, even in the first post-oral.

1 See the Appendix ab the end of the present article, and the accompanyingplate,

568 E. KAY LANKESTER.

The more primitive forms are anomomeristic; the higherforms nomomeristic, showing typically three groups or tag-mata of six somites each.

The genital apertures are placed on the first somite of thesecond tagma or mesosoma. Their position is unknown inthe more primitive forms. The more primitive forms havebranchial respiratory processes developed on a ramus of eachof the post-oral appendages. Iu higher specialised formsthese branchial processes become first of all limited to fivesegments of the mesosoma, then sunk beneath the surface aspulmonary organs, and finally atrophied, their place beingtaken by a well-developed tracheal system.

A character of great diagnostic value in the more primitiveArachnida is the tendency of the chitinous investment of thetergal surface of the telson to unite during growth with thatof the free somites in front of it, so as to form a pygidialshield or posterior carapace, often comprising as many asfifteen somites (Trilobites, Limulus).

A pair of central monomeniscous diplostichous eyes is oftenpresent on the head. Lateral eyes also are often present,which are monostichous with aggregated lenses (Limulus) orwith isolated lenses (Scorpio), or are diplostichous withsimple lens (Pedipalpi, Aranete, etc.).

Class 3 (of the Euarthropoda).—CRUSTACEA.

Head tetartognathous and triprosthomei'ous,—that is to say,with three prosthomeres : the first bearing typical eyes, thesecond a pair of antenniform appendages (often biramose),the third a pair of appendages, usually antenniform, some-times claw-like. The ancestral stock was (as in the Arach-nida) pantoguathobasic,—that is to say, had a gnathobase orjaw-process on the base of every post-oral appendage.

Besides the first post-oral or mandibular pair, at least twosucceeding pairs of appendages are modified as jaws. Thesehave small and insignificant rami, or none at all,—a feature inwhich the Arachnida differ from them. The appendages of

STJUJCTUUE AND CLASSIFICATION OF THE AETHEOPODA. 569

four or more additional following somites may be turnedupwards towards the mouth and assist in the taking of food.

The more primitive forms (Entomosti'aca) are anomomeris-tic, presenting great variety as to number of somites., form ofappendages, and tagmatic grouping; the higher forms (M.ala-costraca) are nomomeristic, showing in front of the telsoutwenty somites, of which the six hinder carry swimmerets,and the five next in front ambulatory limbs. The genitalapertures are neither far forward nor far backward in theseries of somites, e. g. on the fourteenth post-oral in Apus, onthe ninth post-oral in female Astacns and in Cyclops.

With rare exceptions, branchial plates are developed eitherby modification of a ramus of the limbs or as processes on aramus, or upon the sides of the body. No tracheate Crus-tacea are known, but some terrestrial Isopoda developpulmonary in-sinkings of the integument. A characteristiccomparable in value to that presented by the pygidial shieldof Arachuida is the frequent development of a pair of longappendages by the penultimate somite, which, with the telson,form a trih'd, or when that is small a bifid termination tothe body.

The lateral eyes of Crustacea are poly meniscous, with highlyspecialised retinulas like those of Hexapoda, and unlike thesimpler compound lateral eyes of lower Arachnida. Mono-meniscous eyes are rarely present, and when present single,minute, and central in position.

Note.—The Crustacea exhibit a longer and more completeseries of forms than any other class of Arthropoda, and maybe regarded as preserving the most completely representedline of descent.

Class 4.—CHILOPODA.

Head triprosthomerous' and tetartognathous. The twosomites following the mandibular or first post-oral or buccal

1 Embryological evidence of this is still wantiug. In the other classes ofArthropoda we have more or less complete embryological evidence on thesubject. It appears from observation of the embryo that whilst the first

VOL. 4 7 , PART 4 . NEW SERIES. 0 0

570 B. BAY LANKBSTER.

somite carry appendages modilied as niaxillas. The fourthpost-oral somite has its appendages converted into very largeand powerful hernignaths, which are provided with poison-glands. The remaining somites carry single-clawed walkinglegs, a single pair to each somite. The body is anomomeristic,showing in different genera from 17 (inclusive of the analand genital) to 175 somites behind that which bears thepoison-jaws. No tagmata are developed. The genital ductsopeu on the penultimate somite.

Trachese are developed which are dendriform and withspiral thickening of their lining. Their trunks open at pairedstigmata placed laterally in each somite of the trunk or inalternate somites. Usually the tracheae open by pairedstigmata placed upon the sides of a greater or less number ofthe somites, but never quite regularly on alternating somites.At most they are preseDt on all the pedigerous somites except-ing the first and the last. In S c u t i g e r a there are sevenunpaired dorsal stigmata, each leading into a sac, whence anumber of air-holding tubes project into the pericardialblood-sinus.

Eenal CEecal tubes (Malpighian tubes) open into the procto-dseum.

Class 5.—HEXAPODA.

Head shown by its early development to be triprosthonie-rous, and consequently tetartognathous. The first prostho-mere has its appendages represented by the compound eyesand a protocerebrum; the second has the antennas for itsappendages and a deutocerebral neuromere; the third hassuffered suppression of its appendages (which correspondedto the second pair of antennas of Crustacea), but has a trito-cerebrum and coelomic chamber. The mandibular somite

prosthomere of centipedes lias ils appendages reduced and represented onlyby eye-patches (as in Arachnida, Crustacea, and Hexapoda), the second lias arudimentary antenna, which disappears, whilst the third carries the permanentantennee, which accordingly correspond to the second antennae of Crustacea,and are absent in Hexapoda,

STRUCTURE AND CLASSIFICATION 01? TfilS AKTHROrODA. 571

bears a pair of gnathobasic hemignaths without rami or palps,and is followed by two jaw-bearing somites (maxillary andlabial). This enumeration would give six somites in all tothe head; three prosthomeres and three opisthomeres. Eecentinvestigations (Folsom, 4) show the existence in the embryoof a prsemaxillary or supra-lingual somite which is sup-pressed during development. This gives seven somites tothe Hexapod's head, the tergites of which are fused to forma cephalic carapace or box. The number is significant, sinceit agrees with that found in Bdriophthalmous Crustacea, andassigns the labium of the Hexapod to the same somitenumerically as that which carries the labium-like maxilli-pedes of those Crustacea.

The somites following the head are strictly nomomeristicand nomotagmic. The first three form the thorax, theappendages of which are the walking legs, tipped with pairedclaws or ungues. (Compare the homoplastic claws of Scorpioand Peripatus.) Eleven somites follow these, forming theabdominal " tagma," giving thus twenty-one somites in all(as in the higher Crustacea). The somites of the abdomenall may carry rudimentary appendages in the embryo, andsome of the hinder somites may retain their appendages in amodified form in adult life. Terminal telescoping of theabdominal somites and excalation may occur in the adult,reducing the obvious abdominal somites to as few as eight.The genital apertures are median, and placed far back in theseries of somites, viz. the female on the seventh abdominal(seventeenth of the whole series) and the male on the ninthor antepenultimate abdominal (nineteenth of the whole series).The appendages of the eighth and tenth abdominal somitesare modified as gonapophyses. The eleventh abdominalsegment is the telson, usually small and soft; it carries theanus.

The Hexapoda are not only all confined to a very definitedisposition of the somites, appendages, and apertures asthus indicated, but in other characters also they present thespecialisation of a narrowly limited, highly developed order

572 B. RAY IJANKESTER.

of such a class as the Crustacea rather than a range fromlower more generalised to higher more specialised forms suchas that group and also the Arachnida present. It seems tobe a legitimate conclusion that the most primitive Hexapodawere provided with wings, and that the term Pterygotamight be used as a synonym of Hexapoda. Many Hexapodahave lost either one pair or both pairs of wings; cases arecommon of wingless genera allied to ordinary Pterygotegenera. Some Hexapods which are very primitive in otherrespects happen to be also apterous, but this cannot be heldto prove that the possession of wings is not a primitivecharacter of Hexapods (compare the case of the Struthiousbirds). The wings of Hexapoda are lateral expansions of theterga of the second and third thoracic somites. They appearto be serial equivalents (homogeneous meromes) of the trachealgills, which develop in a like position on the abdominalsegments of some aquatic Hexapods.

The Hexapoda are all provided with a highly developedtracheal system, which presents considerable variation inregard to its stigmata or oritices of communication with theexterior. In some a serial arrangement of stigmata compar-able to that observed in Chilopoda is found. In other cases(some larvaa) stigmata are absent; in other cases again asingle stigma is developed, as in the smaller Arachnida andGhilopoda, in the median dorsal line or other unexpectedposition. When the facile tendency of Arthropodato developtracheal air-tubes is admitted, it becomes probable that thetracheae of Hexapods do not all belong to one original system,but may be accounted for by new developments within thegroup. Whether the primitive tracheal system of Hexapodawas a closed one or open by serial stigmata in every somiteremains at present doubtful, but the intimate relation of thesystem to the wings and tracheal gills cannot be overlooked.

The lateral eyes of Hexapoda, like those of Crustacea,belong to the most specialised type of " compound eye,"found only in these two classes. Simple nionomeniscous eyesare also present in many Hexapods.


Renal excretory ceeca (Malpighian tubes) are developedfrom the prootodseum (not from mesenteron, as in scorpionand Amphipoda).

Concluding Remarks on the Relationships to oneanother of the Classes of the Arthropoda.—Out-general conclusion from a survey of the Arthropoda amountsto this, that whilst Peripatus, the Diplopoda, and the Arach-nida represent terrestrial offshoots from successive lowergrades of primitive aquatic Arthropoda which are extinct,the Crustacea alone present a fairly full series of representa-tives leading upwards from unspecialised forms. The latterwere not very far removed from the aquatic ancestors(Trilobifces) of the Arachnida, but differed essentially fromthem by the higher specialisation of the head. We cangather no indication of the forefathers of the Hexapoda or ofthe Chilopoda less specialised than they are, whilst possessingthe essential characteristics of these classes. Neither embryo-logy nor palaeontology assists us in this direction. On theother hand, the facts that the Hexapoda and the Chilopodahave triprosthomerous heads, that the Hexapoda have thesame total number of somites as the nomomeristic Crustacea,and the same number of opisthomeres in the head as themore terrestrial Crustacea, together with the same adaptationof the form of important appendages in correspondingsomites, and that the compound eyes of both Crustacea andHexapoda are extremely specialised and elaborate in struc-ture and identical in that structure, all lead to the suggestionthat the Hexapoda, and with them, at no distant point, theChilopoda, have branched off from the Crustacean main stemas specialised terrestrial lines of descent. And it seemsprobable that in the case of the Hexapoda, at any rate, thepoint of departure was subsequent to the attainment of thenomomeristic character presented by the higher grade ofCrustacea. It is, on the whole, desirable to recognise suchaffinities in our schemes of classification. We may tabulatethe facts as to head-structure in Chastopoda and Arthropodaas follows:

574 E. RAY LANKBSTEli.

Grade x (below the Arthropoda).—AGNATHA APEOSTHOHEEA.

Without parapodial jaws; without the addition of originallypost-oral somites to the pi-seoral region, which is a simpleprostomial lobe of the first somite ; the first somite is per-forated by the mouth, and its parapodia are not modified asjaws. = CH.ETOPODA.

Grade 1 (of the Arthropoda).—MONOGNATHA MONOPEOS-THOMEEA.

With a single pair of parapodial jaws carried by the somitewhich is perforated by the mouth; this is not the first somite,but the second. The first somite has become a prosthomere,and carries a pair of extensile antennas. = ONYCHOPHORA(Peripatus, etc.).

Grade 2 (of the Arthropoda).—DTGNATFA MONOPEOSTHOMEEA.The third somite, as well as the second, develops a pair of

parapodial jaws; the first somite is a prosthomere carryingjointed antenna. = DIPLOPODA.

Grade 3 (of the Arthropoda).—PANTOGNATHA DIPEOSTHOMERA.

A gnathobase is developed (in the primitive stock) onevery pair of post-oral appendages ; two prosthomeres pre-sent, the second somite, as well as the first, having passed infront of the mouth, but only the second has appendages.= AEACHNIDA.

Grade 4 (of the Arthropoda).—PANTOGNATHA TEIPEOSTHOMERA.The original stock, like that of the last grade, has a gnatho-

base on every post-oral appendage, but three prosthomeresare now present, in consequence of the movement of the oralaperture from the third to the fourth somite. The lateral eyesare polymeniscous, with specialised vitrellse and retinulas of adefinite type peculiar to this grade. = CEUSTACEA, OHILO-PODA, HEXAPODA.


According to older views the increase of the number ofsomites in front of the mouth would have been regarded as acase of intercalation by new somite-budding of new prseoralsomites in the series. We are prohibited by a general con-sideration of metamerism in the Arthropoda (see a previoussection of this article) from adopting the hypothesis of inter-calation of somites. However strange it may seem, we haveto suppose that one by one in the course of long historicalevolution somites have passed forwards and the mouth haspassed backwards. In fact, we have to suppose that theactual somite which in grades 1 and 2 bore the mandibleslost those mandibles, developed their rami as tactile organs,and came to occupy a position iu front of the mouth, whilstits previous jaw-bearing function was taken up by the nextsomite in order, into which the oral aperture had passed. Asimilar history must have been slowly brought about whenthis second mandibnlate somite in its turn became agnathousand passed in front of the mouth. The mandibular parapodiamay be supposed during the successive stages of this historyto have had, from the first, well-developed rami (one or two)of a palp-like form, so that the change required when themouth passed away from them would merely consist in thesuppression of the gnathobase. The solid palpless mandiblesuch as we now see in some Arthropoda is, necessarily, a latespecialisation. Moreover it appears probable that the firstsomite never had its parapodia modified as jaws, but becamea prosthomere with tactile appendages before parapodialjaws were developed at all, or rather pa r i passu with theirdevelopment on the second somite. It is worth while bearingin mind a second possibility as to the history of the prostho-meres, viz. that the buccal gnathobasic parapodia (the man-dibles) were in each of the three grades of prosthomerismonly developed after the recession of the mouth aud theaddition of one, of two, or of three post-oral somites to thepraeoral region had taken place. In fact, we may imaginethat the characteristic adaptation of one or more pairs ofpost-oral parapodia to the purposes of the mouth as jaws did

576 K. RAY LANKESTER.

not occur until after ancestral forms with one, with two, and

with three prosthotneres had come into existence. On the

whole the facts seem to be against this supposition, though

we need not suppose that the gnathobase was very large or

the rarni undeveloped in the bnccal parapodia which were

destined to lose their maudibular features and pass in front

of the mouth.

EEPEEENCJKS.

1. BATESON.—'Materials for the Study of Variation' (Macmillan, 1894),p. 85.

2. LANKESTEE.—"Primitive Cell-layers of the Embryo," 'Annals and Mag.Nat. Hist.,' 1873, p. 336.

3. KORSCHELT and HEIDEK.—' Entwickelungsgeschichte ' (Jena, 1892),cap. xv, p. 389.

4. FOLSOM.—" Development of the Mouth Parts of Anurida," ' Bulletin Mus.Comp. Zool. Harvard College,' vol. xxxvi, No. 5, 1900, pp. 142—146.

5. TjANKTiSTisa.—"Observations and Reflections on the Appendages andNervous System of Apus cancriformis," 'Quart. Journ. Micr.Soc.,'vol. xxi, 1881.

6. Hoi'Eit.—"Ein Krebs mit einer extremitiit statt ernes Stielauges," ' Yer-handl. d. Deutschen Zool. Gesellsch.,' 1894.

7. WATASE.—"On the Morphology of the Compound Eyes of Arthropods,"'Studies from the Biol. Lab. of the Johns Hopkins University,'vol. iv, pp. 287—334.

8. BENHAM describes backward shifting of the oral aperture in certain Chreto-pods, 'Proc. Zoolog. Soc. London,' 1900, No. lxiv, p. 976.

N.B.—References to the early literature concerning the group Arthropodawill be found in Carus, 'Geschichte der Zoologie.' The more importantliterature up to 1892 is given in the admirable treatise on Embryology byProfessors Korsclielt and Heider.

STRUCTURE AND CLASSIFICATION OP THE ARTBROPODA. 577

APPENDIX.

ON THE MOVEMENTS OF THE PAEAPODIA OP PERIPATUS, MILLI -

PEDES, AND CENTIPEDES.

[Matter not contained in the article published in the ' EncyclopaediaBritannica.']

I TAKE the opportunity of the issue of my article ' Arthro-poda} as a reprint to add to it some drawings showing themovement of the parapodia or legs oE Onychophora, Diplopoda,and Chilopoda. I was unable to introduce these into theoriginal article, and I now give them in the form of a plate(PI. 42). They were made nearly twenty years ago in mylaboratory at University College, London, from living speci-mens by Miss Stone. The live Peripatus (P. capensis) weregiven to me by Mr. Adam Sedgwick; the Centipede, Seolo-pendra subspinipes (Leach), was brought to me fromBarbadoes by Mr. Tracey; and the Millipede, Archispiros-treptus pyrocephalus (L. Koch), I obtained from Mr.Pocock.

Of course, the attempt to fix and record, by the simple useof eye and pencil, a phase of successional movement, such asthat exhibited by the series of legs of the Arthropoda or ofthe Chtetopoda, is not altogether satisfactory at the presentday. We ought to have records of these phases taken byphotography and the instantaneous illumination of the electricspark. But in the meanwhile the drawings, which were care-fully and conscientiously made after repeated observationand study, show some interesting facts.

A fact which the drawings are not fitted to show is, thatthe change of phase-—that is to say, the alteration in the

578 li. RAY LANKESTEJi.

angle formed by the limb and the long axis of the body,appears to proceed from behind forwards in all three cases.Each leg may be considered as resting normally at rightangles to the axis of the body. Each is capable of a certainforward swing in the horizontal plane, being provided witha joint and muscles at its base, and of a correspondingbackward swing in which the leg passes its first position(that perpendicular to the axis of the body) and makes anexcursion ov deflection away from the perpendicular in theposterior direction. When the animal is in a state oflocomotor activity all the legs steadily swing forwards andbackwards through their extreme range of angular displace-ment, each at the same rate. But they do not all simul-taneously assume the same angular position relatively to theaxis of the body, nor, on the other hand, do they swingirregularly. They pass consecutively from behind forwardsinto an identical phase of the swing movement, the leg- infront taking up the angular phase just previously exhibitedby the leg behind it, which in the meantime has continuedits swinging movement, either becoming more deflected ornow commencing the return movement. The rate of swingis such thab in all cases as yet observed not one great waveoccurs but a series of waves are produced, as when windblows over a cornfield. These waves vary in the number ofunits (legs) involved in a complete wave according to thekind of Arthropod or Chsetopod under observation. Thenumber of units involved in a "wave" or "swing-group"seems to be fixed in a given species, and not to vary accord-ing to circumstances. Whether the rate or relative rate offorward swing is always the same as that of the backwardswing (which is that portion of the swing effective in pro-pulsion) has yet to be ascertained, as also the exact excursionmade on each side of the perpendicular. Also it would beinteresting to ascertain what are the limits of increase anddiminution of the rate of swing, and what nervous mechanism,if any, is concerned in its regulation.

These phenomena can only be studied satisfactorily by

STRUCTURE AND CLASSIFfOATION OP THE ARTflltOPODA. 579

photography, aud require also the consideration of a largenumber of forms, such as a representative series of marineChastopoda, several genera of Diplopoda, and of Chilopoda,the Phyllopod crustaceans and the higher forms, Hexapodinsects and larvee.

The most important fact which the drawings here publishedshow is that in Peripatus and the Millipede the limbs onopposite sides of the body, which are morphologically relatedas " pairs," are always in the same phase of fore-and-aftswing; they move together and identically. On the otherhand in the Centipede the pairs or opposite limbs on asegment are in phases, which are the extreme opposites inthe series of positions through which the limb swings.

Further, it is to be noted in connection with this that thestrongly chitinised body of the Millipede takes no part byserpentine movement in the locomotory process; it remainsperfectly straight. So, too, the soft body of Peripatus—though it is frequently bent and turned on itself, and may bemore or less elongated and contracted at various intervals,yet does not contribute by any serpentine " s t roke" to theprocess of locomotion. On the other hand the Centipede'slocomotion is very largely effected by a powerful lateralundulation of the body—groups of three segments beingalternately slightly tilted by muscular contraction first on oneside and then on the other.

In the case o£ the Centipede, as already noted, thisserpeutine rhythmic movement of the body is accompaniedby an opposition in the phase of the swing movements of thoselegs which are paired with one another in a single segment,and a special kind of leg and body movement is the result,with which the special forms of leg-rhythm producing loco-motion in other highly-developed Arthropoda (including thetripod action in Hexapoda) might be compared with a view toa mechanical explanation of their genesis.

On the other hand it is worth calling to mind that in someof the large marine CliEetopoda, viz. in Nephthys and Nereis(very few observations on the subject have been recorded)

580 E. KAT LANKESTER.

the process of locomotion (when it takes the form of swim-ming) is very definitely assisted by a powerful serpentinemovement of the whole body left and right, whilst the para-podia exhibit a very rapid (far more rapid than in terrestrialwalking Arthropods) swinging action, the phases of whichare identical in the paired appendages of either side of asegment, and not antagonistic in spite of the lateral undula-tion of the body.

One of the important features in the swinging movementof tbe parapodia of Artbropoda and ChEetopoda, which canbe observed by simple inspection of the living animal inmovement, is the fact that the number of pairs of parapodiainvolved in a "swing-group" or (as we may put it) thenumber which one must pass in tracing the phases of move-ment before one comes to a pair of parapodia in exactly thesame phase as that of the pair from which one started, variesin different genera and species. Sometimes the groups maybe represented by a, b, c, d, e, f, g, h, a1, bl, c1, dl, e1,/1, g1, h1,a3, fc2, c2, d2, e2, f, g2, h?, where the letters of the alphabetindicate a parapodium in a given phase of swing, and a in thefirst group is identical in phase with a1 in the second, with as

in the third, and so on. In other cases the groups are repre-sented by two units only—a, b, a1, bl, as, b2, and so on.

In P. capens is (PI. 42, fig. 4) the swing-group number isonly two, a, b. The anterior unit a swings forward, whilstthe posterior unit & has its claws grasping the surface, and isswinging backwards. As soon as parapodium a approachesparapodium bl (and similarly throughout the series) themovement changes, a grasps the surface, and bl (and all theothers corresponding to it, viz. b, &3, bz, b*, &5, be) lets go andcommences to swing forward. This is shown in the figures4, 5, and 6 of PI. 42.

In the Millipede Archispii'ostreptus, on the other hand,the swing-group number is sixteen, and (as our figures 1and 2 of Plate 42 show) there are eight of these groups,allowing for peculiarities in the extreme anterior and posteriorsomites. The regions indicated by the lettering a to / i n the

STRUCTURE AND CLASSIFICATION OF THE AUTHROPODA. 581

figure are regions where parapodia exhibit the extremeforward swing-phase. They may be called " group-crests."Group-crests are but " phases " in the swinging of the limbs,and they pass along the whole series from behind forward,like the crest of a wave passing along a liquid. Eacli pair ofsuccessive parapodia is in turn the seat of the group-crest,and the waves keep flowing from behind forward withbeautiful regularity.

The rate should be measured in different forms, and theconditions affecting the rate of this rhythmic movement shouldbe studied experimentally.

In the Centipede (PI. 42, fig. 3) the " swing-group"number appears to be six, and the whole phenomenon isprofoundly modified by the fact that lateral undulations ofthe body itself are a definite part of the locomotor activity,whilst the limbs on opposite sides of the same segment arenot iden t ica l , but a n t a g o n i s t i c in phase.

It seems to me probable that the condition presented bythe Centipede is a much higher development than that seenin the Millipede, and implies a unilateral differentiation ofmuscles and nerves which is far from primitive. It may, Ithink, be reckoned as one of the characters tending toseparate the Diplopoda or Prosthogoiiopora altogether fromassociation with the Chilopods. It would, of course, be veryinteresting in this connection to have some reliable photo-graphic studies of the phases of parapodial swing in suchforms as Scutigera, and, indeed, in all families of Chilopoda.

EXPLANATION OP PLATE 42,

Illustrating Professor Lankester's article on the Arthropoda.

PIG. 1.—Lateral view of a specimen of Archispirostreptus pyro-cepbalus (de Kocli) drawn from a living specimen in movement. Magnifiedtwice linear.

582 B. l(AY tiA

FIG. 2.—View of the ventral surface of the same specimen crawling on aglass plate and reflected in a mirror. The letters a to h indicate the " group-crests " or extreme phases of forward movement, which traverse the seriesat intervals of sixteen parapodia.

FIG. 3.—Dorsal view of a living specimen of Scolopendra subspinipes(Leach) to show the lateral undulation of the body in locomotion, and thegrouping of the limbs or parapodia in sixes, which are in antagonistic phaseson the two sides of the same segments, but identical with those on the oppo-site side of the next half-group, a, c, e being in the same phase as b, d,f.

PIGS. &—9.—Drawings from live specimens of Peripatus capensis toshow the alternate phases of swing of the parapodia of the same side, and theidentity of the phase of the right and left pairs of one and the same segment,also to show the soft-walled nature of the body, its pliability, and considerablepowers of extension and contraction.

I should be glad were any of my readers able to inform me as to the nameof the author of the following appreciative lines on the subject above dis-cussed.

" A centipede was happy ! TillOne day a toad in fun

Said,' Pray which legMoves after which ?'

This raised her doubts to such a pitch,She fell exhausted in the ditch,

Not knowing how to run."

Slhttri Jau/rn Jhm Set, lf»L 4- 7,KSM 4

< v / / • . ' • / / // /

i I i ' ' : • ' : I i I! I f'T?.i [ i ;;

Archispirosireptus.

9

F i g 1-

! ? 1 ; ! ] • • ) ; 4

Fig 2.

Hulh. Lilk' I.ondoi

Date post:	10-Jun-2018
Category:	Documents
Upload:	lamphuc
View:	218 times
Download:	0 times

The Structure and Classification of the...

Documents