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AUTHORS ABSTRACT OF THIS PAPER ISSUEDB Y THE BIBLIOGRhPHIC SERVICE, JUNE 12

THE NERVOUS SYSTEM OF PHORONIDAWILLIAM A. HILTON

D e p a r t m a t of Zoology, Pomona College, Claremont, Cali forniaSI X FIGURES

It has long been known that the nervous system of Phoronidawas of the surface or epithelial type, such as we find in Echino-dermata, Enteropneusta, and some others. Caldwell (83)was one of the first to describe this condition in Phoronis. Hementions nerve processes in connection with the ectoderm.According to him, both fibers and ganglion cells occur in thesurface. Concentrations of nervous tissue occur about themouth, forming a post-oral nerve ring. This ring forms asemicircular line along the base of the tentacles. Two ciliatedpits occur each side of the middle line, and these are consideredto be sense organs. On the left the nervous system seems to becontinued as a single thick strand near the surface of the bodyjust outside the basement membrane. This rod or strand oftissue Caldwell considers to be a hollow cord. McIntosh ( 88)describes a similar condition in P. buskii; the general form is thesame; the ciliated pits are also recognized; the epithelial nervoussystem has sense cells and ganglion cells and on the left side isthe longitudinal nerve cord or tube, shown but not discussed.A4ndrems(go), in a new species of Phoronis, describes theglandular pit of the lophophore and a large nerve cord on theleft side. He considers it to be solid and surrounded by epider-mal cells. According to him, it seems to have a finely fibrillatedor possibly a coagulated structure. This nerve cord, so-called,connects directly with the left side of the brain or central nervoussystem and runs the length of the body. On the right a similartrunk continues bu t a short distance.

Benham (89) finds the nervous system immediately below theepidermis as Caldwell was first to observe. Nerve cells and381

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382 WILLIAM A. HILTONfibers are recognized in the deeper layers. Some of the fibers arefrom the epithelial cells at the surface. The nervous tissuefollows the epithelium up into the lophophore and into the tenta-cles. Two longitudinal nerve tubes or nerve bands are describedrunning the length of the body. He suggests tha t these bandsmay give the appearance of tubes, due to shrinkage. Epithelialtissue seems to make up these strands, and he did not think theylooked like nervous tissue. The suggestion is made that theseso-called nerves may be sense organs.

Seylys-Longchamps (07) describes the nerve band and thelateral nerve of Caldwell on the left side.Torrey ( l l ) , n Phoronis pacifica, gives a similar account of

the nervous system with th e following exception:The two longitudinal cords which are of exceedingly unequal

length, instead of crossing in the nerve ring of the lophophore,are continuous across the middle line at the level of the medianmass of ganglion cells. The loop thus formed is closely appliedto the latter and touches the lophophore nerve on each side ofthe rectum, apparently without fusing at either point.Pixel1 (12) in P. vancouverensis, describes the usual bandof nerve tissue at the base of the lophophore and the nerves upinto the tentacles. He mentions the large ganglionic mass onthe dorsal surface with the nervous tissue at all places in intimaterelation to the inner ends of epithelial cells. I n some sectionshe finds two small lateral nerve cords along the right and leftsides of the body. These he says are short and composed ofpunctated substance.

Nervous tissue was found in the center of the pit at the proximalend of the body and also along the alimentary canal on the outerside of the epithelium, especially in the region of the oesophagusopposite the nerve ring. This area has been suggested as an organof taste.

In Phoronapsis hameri a similar condition of the nerve ringwas described, but the ring is narrower and more elongated.There is a conspicuous cord down the left side. Its center isclear and on its border nerve cells are described.Harmer (17), in Phoronis ovatus, gives the position andform of the nerve ring which is thickened on the dorsal side.

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NERVOUS SYSTEM OF PHORONIDA 383I have had opportunity of examining a considerable number of

two species of this interesting group. One, Phoronis pacifica,occurs at various places off the coast of southern California,where it has been obtained and examined during three summersThe other, a species of Phoronapsis, has been obtained duringtwo years a t Monterey Bay and one year a t Moro Bay. Bothspecies were studied in serial section. In spite of the occasionaldifficulty with sections due to the presence of sand grains, anumber of perfect series was obtained. Phoronapsis also wasobserved in the field at Monterey and through the kindness ofDoctor Fisher at the Hopkins Marine Station a t Pacific Grove.These forms were also encountered in even greater numbers a tMoro Bay.

In quiet watersthe tentacle whorls are fully expanded just above the surface ofthe sand. Shadows or bright light seem not to affect them insuch positions. Moderate movements in the water made by apipette near caused them to change their positions. Sand grainsdropped gradually upon the expanded tentacles produce littleor no contraction. Touching the tentacles seemed not to bringabout any marked movements, but if the surface was lightlytouched just below the whorl of tentacles there was an immediateand violent reaction and the animal may withdraw within itssandy tube. Any jarring of the sand supporting the sand tubesmay also cause this contraction of tentacles and the withdrawalof the upper portion.

In the laboratory the movements of Phoronapsis were found tobe as follows:1. A contraction of the tentacles and a shortening of theanimal. This is noted whether the animal is inclosed in itssand-grain tube or entirely outside.

2. When out of the tube the whole stem may be waved aboutWithin the tube the bending is limited to a small area near thetentacles, but outside the tube all parts of the body are capable ofbending.

3. Slow contractions when the animal is stimulated outside ofits tube.

The animals do not have many activities.

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384 WILLIAM A. HILTON4.Quick contractions and slower expansion of the tentacles

by separation and by matt ing together.In the laboratory the reactions to light were not apparent, nor

was there any response noted to flashing light. When touchedor jarred the response was noticeable, and in the laboratory whenthe animals were removed from the tubes there was a responseto currents. When specimens were changed in fresh sea-waterthe tentacles tended to separate. This may have been almostpurely a mechanical reaction.

When the tubes are suspended upside down there seems to be alittle reaction to gravity; the stems tend to turn the oral end up.By changing the position, the animals partly extended from thetubes may be caused to rotate the stems through 360".I n general, the tentacles have little power of movement.The whole whorl may expand or contract, but there is littleevidtnt movement of the individual tentacles. When placed insolutions of chloretone the tentacles tend t o separate at once.This takes place a number of times in succession as the animalsare changed back and forth, but in the end they remain expandedand die in that condition, but even when apparently dead insome cases they may at once expand when again placed in thechloretone solution. In solutions which are not fatal, the stemor body seems to be the last to be stopped and the first to recover.Much of the extension of the tentacles is purely a sort of floatingout; a change of water does this a little, but chloretone and evenweak acetic acid may bring about t he same results.

In at least one experiment after chloretone, the power toreally expand the tentacles was recovered. A real response ofthe tentacles after a very weak chloretone solution is rather rare,but I believe it does occur in addition to the purely mechanicalfloating out of the tentacles. When once the animal is killed byweak acid or in some other manner the tentacles no longer floatout in chloretone or in any fluid.

Methylene blue was used in various strengths with the livinganimals and some fairly good results obtained. When notstained too long, bipolar sense cells of a typical sort myere seenin the tentacles and at their bases.

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NERVOUS SYSTEM O F PHORONIDA 385The central nervous system was demonstrated as a more or

less semicircular band which was not very wide. Some scatteredcells in the central nervous system and at the bases of the ten-tacles seemed to be true nerve cells. All over the surface of thebody little groups of stained cells are the sense cells. These aresimilar to those which Retzius describes in some invertebrates.In the body the sense cells were not as well seen as in the tentaclesbecause of the thick opaque regions whore they occurred. Thestain was especially valuable in distinguishing the sensory cellsin the tentacles and in little groups over the surface of' the body.Some nerve strands and cells together with a portion of thecentral nervous system were merely indicated. More intensestain added nothing to the knowledge of the nervous system; onthe contrary, the coloration of connective-tissue cells and othersobscured the results obtained with nerve tissues.The central nervous system of Phoronis pacifica is much likethat described in other forms. It has its chief concentration alittle below the level of the anal opening and the nephridia.This thick par t of the nervous system is directly continuouswith the epithelium of the surface of the body and dorsal to theanal papilla, and about the depression caused by it. From herethe thickening passes towards the tentacles with fibers to themand to the lophophore. Between the depressions on each side arethe chief thickenings of the nervous system. Although thiscentral part is continuous with the epithelium, there are distinctnerve cells and fibers. At this greatest thickening there arethree chief cell centers among the fibers. There is quite athickness of nervous tissue about each depression of the lopho-phore. On the left side near the lophophore is the beginning ofthe clear cord of unknown function first noted by Caldwell. It ispartly surrounded by cells and runs ventrally until it passesthrough the basement membrane of the body-wall and comes tobe just under the epithelium. It runs the length of the body inthis position, becoming smaller and smaller. It does not seemto be of nervous tissue, although it is directly connected withthe nervous system.

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386 WILLIAM A. HILTON

Fig. 1 Section through the body of Phoronis below the tentacles and a t t helevel of the central ganglion. The drawing is not completed on the right, butthe whole extent of the nervous system is shown.

Fig. 2 Section across the body of Phoronis some distance below the levelof figure 1 . The long sense organ or left nerve tru nk is shown on the left below.I n this figure, as in the last, the basement membrane is indicated by a da rk band.X 70 .

X 70 .

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NERVOUS SYSTEM O F PHORONIDA 387

Fig. 3 Section across left side of the junction of the brain and the lophophoreganglion. The brain portion is on the right side of the drawing; the lophophorethickening is a t the left. The beginning of the longitudinal sense organ is nearthe upper par t of the sketch. The basement membrane is shown as a dark band.X 300.

Fig. 4 Section of the longitudinal nerve trunk. X 300.Fig. 5 Another view of a section of the lophophore thickening.Fig. 6 Diagram t o illustrate th e general form of the nervous system in Phoro-

nis. The central nervous system is indicated in the cent ral pa rt of the figure.The left nerve strand or sense organ is shown on the right of the drawing. Thetentacles are shown cut off on the right side. On the left side the nerves to thetentacle bearing ridge and to the tentacles are shown. The mouth and analopenings are not shown. The central nervous system is a mere band of tissue,but the view is such tha t it s greatest bread th and thickness are shown.

X 300.

X 70 .

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388 WILLIAM A. HILTONTorrey ('01) gives a description of the nervous system of this

form from sections, but his rngtnrial must have been poorly fixed,for in well-preserved specimens the central nervous system iscontinuous laterally with the lophophore organs as well as withthe lateral longitudinal cord.

Th e best results from the study of serial sections were obtainedafter Flemming or mercuric chloride fixation followed by one ofthe haematoxylin methods. I n well-preserved specimens thegeneral structure of the neuro-epithelium was well shown.Besides the usual epithelial cell, there were three other typesrecognized : the bipolar sense cells, especially shown in methylenebluc in various parts of the body; the supportive cellz with theslender fibers running through the thickness of the epitheliumand nerve fibers layer, and the nerve cells part ly in the epithelium,partly in the fibrous portion. The fibers run laterally at variouslevels.

CONCLUSIONS1. The nervous system of Phoronis is only partly separated2. There is a distinct center or a central nervous system where3 . The lophophoral organs are well developed and may be sense4. The left longitudinal cord, which may not be nervous tissue,If not

from the epithelium.fibers and cells are more abundant.organs.is distinctly connected with the central nervous system.a nerve structure, i t must be some sort of sense organ.

5. Tentacles and body have bipolar sense cells in li ttle groups.6. The nervous system controls the muscular system asshown by the use of anesthetics. The tentacles recover last andare affected first b y chloretone.

7 . The movements are: a ) Ciliary currents on the tentaclesprobably not under nerve control. 71) Contractions of thetentacles at least partly under nerve control. c) Contractions ofthe body stimulated through the surface of the body at almostany point, especially by tactile stimuli just below the tentacles.

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NERVOUS SYSTEM OF PHORONIDA 389BIBLIOGRAPHY

ANDREWS,E. 1900 On a new American species of the remarkable animalPhoronis. Ann. Mag. Nat . Hist. , 6 ser., vol. 5, pp. 415-449.

BENHAhi, W. B. 1889 The anatomy of Phoronis australis. Q. Jour. Mic. Sc.,CALDWELL, . H. 1883 Preliminary note on the structure, development andCORI,C. J. 1890 Untersuchungen cibcr die Anatomie und Histologie der Gat-DYSTER,. D. 1858 Notes on Phoronis hippodrepia. Trans . Linn. SOC.,vol.HARMER,. F. 1917 On Phoronis ovalis Strethel l. Q. Jour. Mic. Sc., vol. 62,MCIKTOSH,W. C . Notes on Phoronis dredged by H. M. S. Challenger.

Proc. Roy . Soc. Edinb., vol. 11, pp. 211-217.1888 Rep ort on Phoronis buskii, n. sp. , dredged during the voyage ofH. M. S. Challenger.1912 Two new species of Phoronidea from Vancouver Island.Q. Jour . Mic. Sc., N. S. , ol. 58, pp . 257-284, 16 text figs.1901 On Phoronis pacifica sp. nov., Biol. Bull., vol. 2, pp. 283-

V O ~ . 0, pp. 125-158.affinities of Phoronis. Proc. Roy. SOC., ol. 34, pp. 371-383, 1 fig.tung Phoronis. Zcit. f . wiss. Zool., Bd. 49, S. 280-568, pl. 22-28.22, pp. 251-255.pp. 115-148, pl. 7-9.1881

Zool. , vol. 27, part 75, pp . 1-27, pl. 1-3.PIXELL, . L. M.TORREY,. B.288, fig. 1-5.