KEPBODUOTION AND METAMOEPHOSIS 01? COMMON BEL. 373

The Reproduction and Metamorphosis of theCommon Eel (Anguilla vulgaris).1

By

O. B. Grassi,Professor in Home.

FOUR years of continual researches made 1))' me in collabo-ration with my pupil. Dr. Calandruccio, have heen crowned atlast by a success beyond my expectations,—that is to say, haveenabled me to dispel in the most important points the greatmystery which has hitherto surrounded the reproduction andthe development of the Common Eel (Anguilla vulgaris).When I reflect that this mystery has occupied the attention ofnaturalists since the days of Aristotle, it seems to me that ashort extract of my work is perhaps not unworthy to be pre-sented to the Royal Society of London, leaving aside, however,for the present, the morphological part of my results.

The most salient fact discovered by me is that a fish, whichhitherto was known as Leptocephalus brevirostris, is thelarva of the Anguilla vulgaris.

Before giving the proofs of this conclusion I must premisethat the other Mursenoids undergo a similar metamorphosis.Thus I have been able to prove that the Leptocephaluss ten ops (Bellotti), for the greatest part, and also the Lepto-cephalus morrisii and punctatus belong to the cycle ofevolution of Conger vulgar i s ; that the Leptocephalushaeckeli, yarrel l i , bibroni, gegenbauri , kollikeri, andmany other imperfectly described by Facciola, and a part ofthe above-named Leptocephalus stenops of Bellotti,belong to the cycle of evolution of Con gromursena mystax;that the Leptocephalus tsenia, inornatus, and dia-

1 From the 'Proceedings of the Royal Society,' November, 1896.

372 G. B. GEASSI.

phanus belong to that of Congromursena ba lea r ica j thatunder the name of Lep tocepha lus kefers te in i are con-founded the larvae of various species of the-genus Ophich-t h y s ; that the Lep tocepha lus l ong i ros t r i s and theHyoprorus messanens is are the larvse of Ne t t a s tomamelanurum, and that the Lep tocepha lus oxyrhynchusand other new forms are larvae of Saurenche lys cancr i -vora, and that finally a new little Leptocephalus is the larvaof Mursena helena.

The form known as Tylurus belongs to Oxystoma, of whichwe unfortunately know nothing more than a figure by Raffi-nesque. I have not been able to find the Leptocephalus ofMyrus vulgar i s , of which I have had only a single indi-vidual, in which the transformation was already far advanced.Neither have I found the Leptocephalus of Chlopsis b i -color, a very rare form, which is related to Muisena and toM uisenichthys. As the result of these observations, thefamily of the Leptocephalidse has been definitely suppressedby me ; the various forms of that family are, in fact, the normallarvse of the various Mursenoids.

In regard to the greater part of the above-named species,the control has been threefold, namely :

Firstly, anatomical. I have compared the various stages inall their structures, and have made the due allowance for thechanges brought about by the metamorphosis at the close oflarval life.

Secondly, natural. I have found in nature all the requiredtransitional stages.

Thirdly, experimental. I have followed, step by step, themetamorphosis in aquariums.

Therefore the hypothesis of Gunther that the Leptocephaliare abnormal larvse, incapable of further development, must berejected. All this is related by myself at length, with allhistorical details which concern the question, in a largememoir which is about to appear in the journal edited byProfessor Todaro.

Until now all these facts have been unknown, because nor-

REPRODUCTION AND METAMORPHOSIS OF COMMON EEL. 373

mally they can only be observed in the abysses of the sea at adepth of at least 500 metres. Fortunately, along a part of thecoast of Sicily strong currents occur, which must be ascribedto the tide, producing very large displacements of the water inthe narrow Strait of Messina. I shall give further details con-cerning these currents in my large memoir. In consequenceof the strong currents sometimes—I say sometimes becausethere is no regularity, and one may have to wait for a year with-out obtaining any material—not only inany deep-sea fishes, butalso all stages of the development of the Mursenoids are metwith in the surface-water. To these currents we owe all thecaptures of Mursena he lena with ripe eggs, which is in ac-cordance with what I had already argued from other facts,namely, that the reproduction of the Murtenoids takes place atgreat depths of the sea.

Before I proceed to speak of the common eel, I must pre-mise that Dr. Raffaele has described certain pelagic eggs asbelonging to an undetermined species, putting forward thesuggestion that these eggs belong to some Mursenoid. Thismatter has been investigated by myself, and I have shown thatthe newly hatched larvae (called " prse-larvae" by me) derivedfrom these eggs have essentially the character of Leptocephali.

The life history of the Muraenoids, leaving aside for thepresent the common eel, is as follows:—Females can onlymatute in very profound depths of the sea, that is to say, atleast a depth of 500 metres. This fact I established by find-ing well-known deep-sea fishes together with Leptocephali,ripe Mursense, and quite ripe eels (see below). The females ofthose species which do not live at this depth must thereforemigrate to it. The male, however, can mature at a smallerdepth, and therefore they migrate into the greater depth whenthey are already mature. Fertilisation takes place at greatdepths : the eggs float in the water; nevertheless they remainat a great depth in the sea, and only exceptionally, for un-known reasons, some of them mount to the surface.

From the egg issues rapidly a prse-larva, which becomes alarva (Leptocephalus) with the anus and the urinary opening

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374 G. B. GEASSI.

near the tip of the tail. The larva then becomes a hemi-larva,the two apertures just named moving their position towardsthe anterior part of the body, which becomes thickened andnearly round. By further change the hemi-larva assumes thedefinitive or adult form. The larva, as well as the hemi-larva,shows a length of body much greater than that exhibited bythe young Muraenoid of adult form into which they are trans-formpd. By keeping specimens in an aquarium I was able toestablish a diminution of more than 4 cm. during the meta-morphosis. With regard to the greatest length which the larvacan attain in a given species, and the amount of diminutionwhich accompanies metamorphosis, there are great individualvariations.

The history of the common eel, to which I am now aboutto refer, is very similar to that given above for the otherMurEenoids. The common eel (Angui l la vu lgar i s ) under-goes a metamorphosis, and before it assumes the definitiveadult form it presents itself as aLeptocephalus, which is knownas Leptocephalus brevi ros t r i s . This Leptocephalus wasdiscovered in the Straits of Messina many years ago. A speci-men was also captured by the " Challenger," and anotherspecimen was taken by the Zoological Station of Naples in theStraits of Messina. This form is occasionally carried to thesurface by currents. By exception, in the month of March, inthe year 1895, we captured several thousands of them in oneday; but the best way to secure this Leptocephalus (and a veryeasy one) is to open the intestine of the Orthagoriscusmola, a fish which is common in the Straits of Messina, andin it one is certain to find a very large number of specimens.It must be observed that Or thagor i scus mola is a deep-seafish. The specimens of Leptocephalus brevirostr is foundin the intestine of Orthagoriscus are more or less altered bydigestion. Those specimens of Leptocephalus breviros t r iswhich are taken near the surface in the open sea are in a betterstate of preservation, but, unfortunately, these also frequentlyhave the epidermis injured so that they cannot maintain theirlife in an aquarium for more than a few days; they live long

BEPEODUCTION AND METAMOBPHOSIS OF COMMON EEL. 375

enough, however,, to allow us to observe that it is their habitto conceal themselves in the sand or in the mud as the commoneel (Anguilla) does. Here it is to be noted that the variousforms of Leptocephali have habits resembling those of theMursenoids to which they belong, i.e. they dig into the sandor abstain from doing so according as the adult form has or hasnot this habit.

I now pass on to the characters o fLep tocepha lus brevi-ros t r i s . I give them here in the same order as I shall use inmy larger memoir. The length varies from 77 to 60 mm., thesame extent of variation as observed in other Mursenoids.The caudal fin tends to assume the form which it has in theElver 1 or young Anguilla. It is to be noted that in otherLeptocephali the caudal fin also tends always to exhibit theadult form. The lower jaw projects sometimes more than theupper jaw, as in Anguilla. The margin of the mouth is wide,as in Anguilla. The tongue is free, as in Anguilla. On theother hand, the youngest elvers which 1 have observed havesmaller eyes than Lep tocepha lus brev i ros t r i s , and thisneed not surprise us, since we know that in other species ofMurgenoids the diminution of the eyes occurs during themetamorphosis. The nostrils are separated from one another,the anterior tubes are relatively at a considerable distance fromthe tip of the snout and from the rim of the mouth. Theyare in a position in which they are observed in many otherLeptocephali, which are destined to transform themselves intoadult forms having the anterior nostrils in nearly the sameposition as in the common eel. The posterior nostrils, on thecontrary, are not tube-like, and are in the same position asthose occupied in the adult Anguilla. It is worth remarkingthat in other Leptocephali also the posterior nostrils havealready assumed the adult position when the anterior ones arestill far removed from it. In L. brevi ros t r i s I find a larvaldentition which resembles that of the other Leptoce-

1 The word " Elver " is used in this paper in its strict sense, viz. for theyoung form of Anguilla vulgar is as taken when ascending rivers in vastnumbers.

376 a. B. GEASST.

phali. In] correspondence with the small size of Lep to-cephalus b rev i ros t r i s the number of larval teeth issmall. Researches founded, firstly, on the enumeration ofthe myomeres; secondly, upon the enumeration of the dorsaland ventral arches of the vertebrae of the caudal extremity(hypurals) j and thirdly, upon the enumeration of the posteriorspinal ganglia, lead with great certainty to the conclusion thatthe Lep tocepha lus brevi ros t r i s is the larva of a Mursenoid,the number of whose vertebrae must lie between 112 and 117,most probably 114 or 115. Such a Mursenoid is the Anguil lavulgaris . The Mursenoid indicated cannot be any other ofthose occurring in the Mediterranean, because they all have anumber of vertebrae higher than 124.1 Counting the myomeresin Leptocephalus brevirostr is , one finds generally only 105complete, five others incomplete, and all the others in a stateof transparency and incomplete formation. These latter arefortunately at the posterior extremity, where other criteriacome to our assistance, namely, the spinal ganglia and thevertebral arches. To show how I arrive at the number ofvertebrae which must be possessed by the adult Individual,corresponding to a given Lep tocepha lus b rev i ros t r i s , Iquote the following example:—I assume that three vertebraedevelop themselves in correspondence to the first four incom-plete myomeres, and that 105 must develop themselves inrelation to the 105 complete myomeres,—that is to say, betweenthe fourth and fifth myomeres, between the fifth and sixth, andso on until we reach the 105th vertebra lying between the104th and 105th myomeres. I further conclude that sevenother vertebrae are developed at the caudal extremity, as indi-cated by the number of vertebral arches and the spinal gangliain that region. We count, therefore, in all 115 vertebrae, andthis is the number which can be easily seen in many specimensof Angui l la vu lgar i s .

1 Muraenesox savanna is said to have 109 vertebrae, but it isdoubtful whether it really occurs in the Mediterranean. The position ofits nostrils and the number of its branchiostegal rajs render its associationwith Leptocephalus brevirostris impossible.

itEPBODUOTION AND METAMORPHOSIS OP COMMON EEL. 377

Here I must particularly insist that I have ascertained inan absolute manner that during the metamorphosis of theMursenoids the number neither of the myomeres, nor of thevertebral arches, nor of the spinal ganglia is subjected to anychange. The hypurals of Leptocephalus b rev i ros t r i s areprecisely the same as in the elver of Angui l l a vu lgar i s .The last hypural which is fused with the urostyle may pre-sent itself as a single piece, or may be more or less cleft.These are variations which are met with also in the elver.Just as in the elver, the last hypural but one is always ex-tensively cleft, or, if the expression is preferred, doubled. Tothe last hypural correspond five rays, whilst four correspond tothe last but one, and one to the last but two, the wholestructure being identical with that found in the elvers ofAngui l la vulgaris . Of these ten rays, the eighth, seventh,and sixth are bifid, both in Leptocepha lus brevi ros t r i s andin the elvers of Angui l l a vulgaris . In the pectoral fin ofLeptocephalus b rev i ro s t r i s the definitive rays can be ob-served, and these are of the same number as in the elvers of An-guil la vulgaris . Lep tocepha lus brevi ros t r i s is transpa-rent, and has colourless blood. The red corpuscles are wanting,but there are present so-called "blood-plates" ("Blutplattchen"in German) similar to those of the inferior Vertebrates. Thebile is also colourless. This fact is observed in all the otherLeptocephali. Leptocephalus b rev i ros t r i s is, however,the only one which is free from all pigmentation. Corre-spondingly, the common eel is the only species of Mursenoidwhich at the close of metamorphosis (i.e. in the youngestelvers) is devoid of all trace of larval pigmentation. It wasthis observation which first led us to the discovery of therelations between Leptocephalus b rev i ros t r i s and An-guilla vulgaris.

In making transverse sections of Leptocephalus brevi-rostris I found other characters which confirm the relationbetween it and the common eel; for instance, the branchio-stegal rays are ten to eleven in number, as is also observed inthe elvers of Anguilla vulgaris. In the common eel the

378 G. B. GEASSI.

well-known lateral branch of the fifth pair of the cranialnerves exists. It is also found in Lep tocepha lus brevi-rostr is . This lateral branch could not be found by Dr.Calandruccio in the other common Mursenoids of Sicily, andis wanting also in the other Leptocephali.

The mucous canal-system (sensory canals) in the head arealready developed, partially in Leptocephalus brevi-rost r is , and are incompletely developed in the elver. As inthe elver, so in Lep tocepha lus b rev i ros t r i s , the pyloriccaeca are wanting. The blind extremity of the stomach andthe incompletely developed swim-bladder, which is as yet freefrom contained gas, are present both in Leptocephalusb rev i ros t r i s and in the elver of Angui l la vulgar is . Thepronephros is in active function, as in the other Leptocephali.The Malpighian glomerules of the kidney (mesonephros) arelobed as in the eel, and their number corresponds with thatobserved in the Helmichthys stage, of which I will speakfurther on. The genital gland, not yet sexually differentiated,is almost identical with that of the same stage. In short, itmay be said that the whole organisation of Leptocephalusbrev i ros t r i s corresponds with the organisation of the commoneel, if we make allowance for those changes which areobserved in the metamorphosis of the other species of Murse-noids, such as reduction of the pancreas and of the liver, dis-appearance of the protoskeleton, complication of the muscu-lature, increase in size of the cerebellum, loss of the larvalteeth, development of the definitive teeth, &c.

From the description of these Leptocepbali I must pass onbriefly to speak of the stages nearer to the condition of theelver. I am, however, obliged to leave a. break in the series,which, however little its significance, yet certainly will makesome impression on the minds of those who do not realise with,what caution I have formed my conclusions. I must confessthat since I have learnt how difficult it is to procure an entireseries of the development of a Mursenoid, I am more astonishedat being able to recognise a single stage in the development ofa given species than at not finding the whole series, I must

fiUPKOnUCTION AND METAMORPHOSIS OP COMMON EEL. 379-

point out that the break in my series of the development ofA n g u i l l a vulgar is would have been much smaller if I couldhave persuaded myself to kill and preserve one of the hemi-larvse which I happened to meet with at the end of the year1892. They were really transitional stages between Lepto-

Fia. 1.

FIG. 2.

FIG. 3.

FIG. 4.

FIG. 1,—Leptocephalus brevirostris. Natural size.FIG. 2,—Lep tocephalus brevirostris. Later stage. Natural size.FIG. 3»—Anguilla vulgaris. Transition stage. Natural size.FIG. 4.—Anguilla vulgaris. Definitive habit (Elver). Natural size.

cephalus breviros t r is and that stage which I shall describefurther on. I published this fact in a preliminary note in themonth of May, 1893. They were transparent with almostcolourless blood, without any trace of pigmentation except atthe eyes, and had lost all the larval teeth, whilst they possessedalready very few and very minute teeth of the definitive series.

380 G. B. GBASST.

The body was thickened, and already showed the cylindricalform. They measured little less than 8 cm. In short, theywere Leptocephalus b rev i ros t r i s on the ray to trans-formation into Angui l la vulgar is. As a matter of historythey actually did transform themselves in my aquarium withthe usual diminution in their dimensions, and subsequentlyproceeded to increase in bulk.1 The metamorphosis tookplace, as usual, without the animal taking in any nourish-ment whatever. The resumption of growth was accompaniedby a resumption of feeding. Unfortunately I had no otherindividuals of this stage.

The stage which I now pass on to describe (fig. 3) canbe obtained during the winter in the sea. I have neverfound them at the mouths of rivers. The length variesfrom 54 to 73 mm. Most individuals measured about 65 mm.The body is relatively longer than in the elver. It is alsorelatively deeper, as in Leptocephalus. We are'remindedof Leptocephalus also by the pigment of the eye, the vitreoustransparency of the body, the swim-bladder being indis-tinguishable in the living animal, and the absence of alllarval pigmentation. The blood is slightly coloured, and thebile is already green. Slight pigmentation can be seen alongthe central nervous system, and at the middle part of thecaudal fin. This commencement of the definitive or adultpigmentation in the regions named before it occurs in anyother part is also seen in other Mursenoids. The definitiveteeth are very minute, and few in number. The intestinecontains no food. After what I had observed in the otherMurzenoids, the simple observation of the barely indicatedteeth and of the absence of aliment in the gut, would havebeen sufficient to convince me that the stage now under noticemust be preceded by a Leptocephalus phase. Indeed, if wedid not admit such a preceding history, we could not under-stand how this little fish could have attained such a size with-

1 The fact that I actually have obtained in an aquarium the transforma-tion of L. brevirostris iuto Anguilla vulgaris is of prime importance.The time occupied was one inontli.

REPRODUCTION AND METAMORPHOSIS OF COMMON EEL. 381

out acquiring well-developed teeth, and without nourishingitself.

In conclusion, no one would hesitate, even not knowingLeptocepTialus brevirostris, to refer the stage now underdiscussion to a Mursenoid about to complete its Leptocephalusmetamorphosis, were it not for the fact that there has been somuch question concerning the reproduction of the commoneel, and that so many capable observers have failed in dealingwith it, that every new observation is received with scepticism.The stage of which I am now speaking, in the hands of apure systematise would probably be described as a Helmichthys,a genus established for certain forms of Leptocephali faradvanced in transformation.

The next forms to which I have to refer are captured in thecourse of migration from the sea into fresh water. When keptin an aquarium they assume the characters of the elver, dimin-ishing more or less in volume, and without nourishing them-selves. The elvers of the common eel can present themselvesin stages differing little from that last described, as well as ina form which has already developed the full pigmentation ofthe adult. Even those which most resemble the precedingstage always have a character which distinguishes them easily,namely, the presence of a definitive pigment, more or lesssuperficially placed on the head, and not to be confoundedwith the pigment round the posterior extremity of the brain,which latter is already present in the preceding stage. Inspecimens taken at the mouths of rivers this more or lesssuperficial pigment was, so far as I could ascertain, alwayspresent.

As the pigmentation develops itself, the little eel graduallyundergoes a diminution in all its dimensions. It results frommy measurements, that the fully pigmented elver has anaverage length of 61 mm., while for the more or less colourlesselver the average length is 67 mm. I found pigmented elverswhich were reduced in length to 51 mm., a size which I neverobserved in those elvers in which the development of pig-ment had not taken place.

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382 G. B. GEASSI.

The facts which I have stated demonstrate that the eel goesthrough a metamorphosis, and that Leptocephalus brevi -rostris is its larva. Some further considerations remain tobe given, although I believe that zoologists will not considerthe question still an open one after the record of facts givenabove—facts which any one may verify by examining thematerial which is preserved in my hands. Many to whom Ihave related my discovery of the history of the common eel•have objected that eels are found almost everywhere, whilstLeptocephalus brevi ros t r i s is limited to Messina. Inreply, I must say that, first of all, it is not true that Lepto-cephalus brevi ros t r i s is limited to Messina; secondly, thatat Messina there are special currents, which tear up the deep-sea bottom which everywhere else is inaccessible; thirdly,although it is true that on the coasts of many countries whereAngui l la vu lgar i s is found, no one has ever seen a Lepto-cephalus b r e v i r o s t r i s ; it is also true that in no country,not even in those where eels are abundant, has anyone everseen an eel of less than 5 cm. in length. Since it has to beadmitted that no one knows the eel before it arrives at thelength of 5 cm., there is no greater difficulty in supposing thatduring this unknown period the eel passes through a Lepto-cephalus stage than in supposing that it does not do so. Thecritical study of the literature of this subject, and a great manycontinued observations, have occupied me for many years, andhave been undertaken just in those places where young eelsare to be found. They enable me, from my own studies, toaffirm with assurance that young eels with the definitive adultform do not exist of less than 5 cm. in length.

From the study of the memoir of Raffaele on pelagic eggs, Ihave come to the conclusion that the eggs of his undeterminedspecies No. 10, having a diameter of 2'7 mm., and differingfrom all the others in the absence of oil-globules,1 must belongto the Angui l la vulgar is , because from them Dr. Raffaele

1 Renewed researches have convinced me that this egg is that of An-guilla vulgaris. There is, however, another egg belonging to an unde-termined Murtcnoid which is devoid of oil-drops, and can easily be confusedwith the true eggs of Anguilla.

REPRODUCTION AND METAMORPHOSIS OF COMMON EEL. 383

obtained prse-larvse which had only forty-four abdominalmyomeres. I endeavoured for two years in vain to studythese eggs at the Zoological Station of Naples. I found onlya few of them, and these died prematurely.

In another point my researches have yielded a very inte-resting result. As a result of the observations of Petersen,we know now that the common eel develops a bridal colora-tion or " mating habit," which is chiefly characterised by thesilver pigment without trace of yellow, and by the more orless black colour of the pectoral fin, and finally by the largeeyes. Petersen inferred that this was the bridal colorationfrom the circumstance that the individuals exhibiting ithad the genital organs largely developed, had ceased to takenourishment, and were migrating to the sea. Here Petersen'sobservations cease and mine begin. The same currents atMessina which bring us the Leptocephali bring us also manyspecimens of the common eel, all of which exhibit the silvercoloration. Not a few of them present the characters de-scribed by Petersen in an exaggerated condition,—that is tosay, the eyes are larger and nearly round instead of elliptical,whilst the pectoral fins are of an intense black. It is worthnoting that in a certain number of them the anterior marginof the gill-slit is intensely black, a character which I havenever observed in eels which had not yet migrated to the sea,and which is wanting in the figures and in the originals sent tome by Petersen himself. Undoubtedly the most important ofthese changes is that of the increase of the diameter of the eye,because it finds its physiological explanation in the circumstancethat the eel matures in the depths of the sea. That, as amatter of fact, eels dredged from the bottom of the sea havelarger eyes than one ever finds in fresh-water eels I haveproved by many comparative measurements, made between eelsdredged from the sea bottom and others which had not yetpassed into the deep waters of the sea. Thus, for instance, ina male eel taken from the Messina currents, and having a totallength of 3 4 | cm., the eye had a diameter, both vertical andtransversal, of 9 mm. and in another eel of 33£ cm. the same

384 G. B. GEASSI.

measurement was recorded. In a female eel, derived from thesame source and purchased in the market, whose length was48J cm., the vertical diameter of the eye was 10 mm., andthe transversal diameter rather more than 10 mm. Theseare not the greatest dimensions which I observed, and I con-clude from these facts that the bridal habit described by Petersenwas not quite completed in his specimens, and that it becomesso only in the sea and at a great depth. In relation to theseobservations of mine stands the fact that the genital organs inthe eel taken in the Messina currents are sometimes moredeveloped than in eels which have not yet entered the deepwater. Thus it has happened that male individuals haveoccurred showing in the testes here and there knots of sper-matozoa. These spermatozoa are similar to those of theConger vulgar i s , and must be considered as ripe. As iswell known, so advanced a stage of sexual maturity has neverbefore been observed in the common eel. This appears to bedue to the fact that the males hitherto examined had not yetmigrated into the deep water of the sea.

Eels with big eyes taken from the depths of the sea were,before the above facts were known, described as a distinctspecies under the name of A n g u i l l a bibroni (Kaup) and ofAngui l la k iene r i (Kaup), not to be confounded with An-gu i l l a k iener i (Giinther), which is a synonym of Lycodeskieneri .

In certain cloacse of ancient Rome which to-day are disusedand contain pure water, remarkable eels are found of a lengthof from 20 to 30 cm., of a grey colour, without trace of yellow,of male and female sex, with enormous eyes, and with more orless rudimentary genital organs. They are individuals which,confined in a place without light, have acquired prematurelyone of the characters of the bridal habit without a correspond-ing development of the genital organs. These individuals areprobably incapable of ulterior development, as the conditionof their genital organs seems to demonstrate.

Under the name Angu i l l a k iene r i (Kaup) there haveprobably been included some individuals which had acquired

BBPRODTTCTION AND METAMOKPHOSIS OF COMMON EEL. 385

big eyes under conditions similar to those described for theeels of these Roman cloacee. From these and similar observa-tions it clearly results that all the European eels must be in-cluded under a single species; and this is an important factfrom another point of view, namely, that it destroys anobjection which might be raised against my conclusion withregafd to the development of A n g u i l l a vu lgar i s from L e p t o -cepha lus b rev i ros t r i s , namely, the objection that Lep to -cepha lus b rev i ros t r i s belongs not to A n g u i l l a v u l g a r i s ,but to Angui l l a k iener i , or to A n g u i l l a b ib ron i .

To sum u p , A n g u i l l a v u l g a r i s , the common eel, maturesin the depths of the sea, where it acquires larger eyes than areever observed in individuals which have not yet migrated todeep water, with the exception of the eels of the Romancloacae. The abysses of the sea are the spawning-places of thecommon eel: its eggs float in the sea water. In developingfrom the egg it undergoes a metamorphosis, that is to say,passes through a larval form denominated L e p t o c e p h a l u sbrev i ros t r i s . What length of time this development re-quires is very difficult to establish. So far we have only thefollowing data:—First, A n g u i l l a vu lga r i s migrates to thesea from the month of October to the month of January;second, the currents, such as those of Messina, throw up, fromthe abysses of the sea, specimens which, from the commence-ment of November to the end of July, are observed to be moreadvanced in development than at other times, but not yetarrived at total maturity; third, eggs, which according toevery probability belong to the common eel, are found in thesea from the month of August to that of January inclusive;fourth, the L e p t o c e p h a l u s b rev i ros t r i s abounds fromFebruary to September. As to the other months, we are insome uncertainty, because during them our only natural fisher-man, the O r t h a g o r i s c u s mo la, appears very rarely; fifth,I am inclined to believe that the elvers ascending our riversare already one year of age, and I have observed that in anaquarium specimens of L. b r e v i r o s t r i s can transform them-selves into young elvers in one month's time.

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