the development of ophiocoma nigra

The Development of Ophiocoma nigra.By

N. Narasimhamurti, Ph.D., D.I.C.(Marshall Scholar.)

Imperial College of Science and Technology, London.

With Plates 5 and 6.

INTRODUCTION.

THE present work, which has occupied my attention for thelast two years, was undertaken at the suggestion of ProfessorE. W. MacBride, P.R.S. A reinvestigation of the developmentof an Ophiuroid was thought to be necessary, as the only com-plete work on the subject was done by Professor MacBridehimself working on O p h i o t h r i x f ragi l i s as far back as1907. As he had some doubts with regard to some of his inter-pretations he asked me to go down to the Marine BiologicalLaboratory at Plymouth and work on Ophiocoma n i g r a ,which is found in large numbers off the coast of Plymouth.

PREVIOUS HISTORY.

We owe our first knowledge of the development of Ophiuroidsto Johannes Miiller (1845-1846), who whilst visiting Heligolandhappened to observe a larva of an Ophiuroid and named it'pluteus paradoxus'. In a subsequent paper he described themetamorphosis of this pluteus paradoxus and showed that itwas the larva of an Ophiuroid. However, he could not interprethis results fully and failed to identify the coelomic rudiments.In a later paper (1851) he described two further forms ofOphiuroid larvae, one of which he recognized as the larva ofO p h i o t h r i x f r ag i l i s . In the other, which he called the'pluteus bimaculatus', he discovered the larval anus ofOphiuroids. Further, he described the rudiment of the water-vascular system as the 'palmate organ', since it possessed fivefinger-like outgrowths, but was led astray by the five-lobed

64 N. NARASIMHAMURTI

condition of each primary lobe into imagining that the palmateorgan of the earlier larva was identical with the first of theselobes. Miiller (1852) also described a worm-like larva of anOphiuroid. In 1900 Caswell Grave found a similar larva onthe American coast and identified it as that of Oph iu rab r e v i s p i n a .

The next reference to Ophiuroid development is found in thework of Metschnikoff (1869). Two kinds of plutei were describedby him. He was able to obtain only one form with sufficientstages to describe the development in some detail. He dis-criminated the right and left coelomic rudiments, and the five-lobed hydrocoele which later encircled the oesophagus. Hecorrected Miiller's error about the palmate organs, and showedthat, unlike Echinoids, the larval oesophagus persisted in theadult Ophiuroid. He further described the rotation of themadreporic pore to the right side. In this paper Metschnikoffgives valuable information with regard to the viviparous speciesA m p h i u r a s q u a m a t a . In A m p h i u r a he described thedivision of the coeloms into right and left anterior, and rightand left posterior coeloms, and the assumption of a five-lobedform by the anterior cavity. He made an important discovery,that the right anterior sac sometimes assumed a rosette-form,and here we find the first definite hint of the paired hydrocoelerudiment.

Further data on the development of A m p h i u r a s q u a m a t aare provided by Ludwig (1881), Apostolides (1882), Fewkes(1886), Carpenter (1887), Eusso (1891), and MacBride (1892).Ludwig's work (1881) referred to the development of the skeleton,and established the homology of the so-called vertebra with apair of the ambulacral plates of an Asteroid. Apostolides (1882),who worked out the development of this species, put forward theextraordinary view that the endoderm was formed by delamina-tion. Eusso (1891) corroborated this statement and added thatthe coelomic cavities arose as spaces in the mesenchyme. Fewkes(1886) dealt chiefly with the skeleton, but asserted the ectodermalorigin of the oesophagus. Carpenter's (1887) work is ratherunsatisfactory owing to his attempt to homologize the dorsalplates of A m p h i u r a with the ahoral plates of a crinoid.

DEVELOPMENT OF OPHIOCOMA 65

MacBride (1892) described mainly the post-larval stages andshowed that the so-called heart, a problematical organ lyingnear the stone canal, represented the first rudiment of the genitalcells. He discriminated between various spaces confused underthe name of the 'axial sinus'. He called the space which heidentified later with the right water vascular rudiment' sinus B'.

The next great advance in our knowledge of Ophiuroiddevelopment was made by Bury (1889). He revised the ideaof a metameric segmentation of the Echinoderm larvae. Inhis paper (1889) on the embryology of Echinoderms, he clearlydemonstrated the transverse division of the coelom in the larvaeof both Echinoids and Ophiuroids. He distinguished betweenthe left anterior coelom and the left hydrocoele. This latter,he believed, arose from the left anterior coelom in Echinoids,but was compelled to assert that it originated from the leftposterior coelom in Ophiuroids In a later paper, Bury (1895)gave a complete account of the relationship between the variousgroups of Echinoderms, and made a valuable suggestion thatin Ophiuroids during metamorphosis the left posterior coelomencircled the stomach.

Zeigler (1896) published a short paper on the early develop-ment of O p h i o t h r i x f r ag i l i s . The only important dis-covery of his was that the early larva is characterized by avacuolated crest.

The next important work in this direction was by Grave(1900) on Oph iu ra b r e v i s p i n a . Because of a cellular plugextending from the apex of the coelom into the archentericcavity he fancifully concluded that the archenteron was formedby the hollowing out of the inner mass of the larva. He clearlyshowed that the bi-lobed coelomic vesicle was constricted off thearchenteron and later divided into right and left halves. Hemade an extraordinary discovery: that a second coelomic vesiclewas budded from the archenteron and that it subsequentlybecame divided by constriction into two, the anterior halfgiving rise to the left hydrocoele while the posterior becamethe left posterior or hypogastric coelom. This notion about theindependent origin of the left posterior coelom is unique, andbears a resemblance to the state of affairs in the star-fish

NO. 301 F


Cr ibe l l a , the development of which has been worked outby Masterman (1902).

Mortensen (1898 and 1900) made a systematic review of allthe varieties of Echinoderm larvae. He was the first to introducethe terms ' Echinopluteus' and ' Ophiopluteus', thus distin-guishing the two types of larvae.

The latest important work on the development of Ophiuroidsis by MacBride (1907) on O p h i o t h r i x f r ag i l i s . Hedefinitely established the'origin of the coelom as a single vesiclefrom the apex of the archenteron, and its later division into threesegments on each side. He asserted that the middle rightsegment, which occasionally assumed a five-lobed form, was theright antimere of the water vascular system. He clearly demon-strated the origin of the hydrocoele as well as the posteriorcoelom from the anterior coelom. He described the origin ofsome of the perihaemal spaces and the perioral coelom from theleft posterior coelom, which had assumed a ring shape in thecourse of metamorphosis. He showed that the larval oesophaguspersisted, and that the intestine degenerated, and describedthe origin of the primitive germ-cells from the wall of the leftposterior coelom. In short, MacBride's paper gives a completeand satisfactory account of the formation of organs in anOphiuroid.

MATERIAL AND METHODS.

In the summer of 1930 I went to Plymouth to try to rearOphiocoma n i g r a , and obtain all the developmentalstages. Eipe males and females were obtained, and the eggswere fertilized. The resulting larvae were introduced into aBrowne plunger jar. The larvae were exceedingly vigorous anddeveloped with great rapidity. The question of food for thelarvae was the greatest trouble which was encountered. UnlikeEchinoplutei and Bipinnariae, it was found the Ophiuroid larvaedid not consume the diatom Nitschia to any appreciable extent,although a few diatoms were found in the stomachs of laterlarvae. Fortunately, in the sea-water contained in the plungerjar there was an unusual amount of phytoplankton, and thisprobably served as nourishment to the growing larvae, which


completed metamorphosis in about forty days. The number ofmetamorphosing stages of the larvae obtained was ratherscanty, but plenty of late larvae were obtained from theplankton towards the end of my stay at Plymouth. The larvaeof Ophiocoma n ig ra were easily distinguishable by theloop at the junction of postero-lateral and post-oral arms. Theapproximate time for the appearance of various organs are givenin the following table:Free-swimming blastula . . . . 24 to 30 hours.Gastrula . . . . . . . 40 to 48 hours.Formation of coelom . . . . . 50 to 60 hours.Stomodaeum meets the gut . . . . About 3 days.First transverse division of coelom . 6 to 8 days.The appearance of hydrocoele . . . 6 to 8 days.Left hydrocoele 5-lobed . . . . 2 5 days.Formation of hydrocoele ring round oesophagus 30 to 35 days.Degeneration and absorption of larval arms . 35 to 40 days.

These times are more or less approximate for artificial cul-tures. The larvae obtained from plankton were, however, largerand healthier, the rate of development varying with the amountof food present. -

The larvae were preserved in osmic acid followed by Miiller'sfluid and Bouin Duboscq. The latter fixation gave splendidresults. A few of different stages of development were preservedin 40 per cent, formaldehyde and transferred to 90 per cent,alcohol for making whole mounts of larvae in order to supple-ment the observations made on living plutei.

For sectioning purposes the celloidin in paraffin method wasused. Most of the sections were cut parallel to the frontal planeof the larva. A few sagittal and transverse sections were alsoemployed when necessary to elucidate special points.

EARLY LAEVAL DEVELOPMENT.

Cleavage , B l a s t u l a t i o n , and G a s t r u l a t i o n ,The eggs of Ophiocoma are small, being about 0-1 mm.

in diameter, and opaque, owing to the presence of yellowish-brown yolk. During fertilization normally a single spermatozoon

F 2


enters the egg and unites with the egg-nucleus. The membraneof fertilization appears immediately after the entrance of thesperm. Its formation begins at the point where the spermato-zoon entered the egg. At this point there is a slight change ofshape in the surface of the egg. The membrane completelyseparates from the egg and stands off at a distance of about halfof the diameter of the egg itself.

Cleavage does not occur as rapidly as MacBride describesin O p h i o t h r i x f r ag i l i s . Blastula formation occurs onlyabout twelve hours after fertilization. The first cleavage occursin the vertical plane dividing the egg into two equal parts.Next another vertical division divides it into four cells ofapproximately equal size. The succeeding cleavages are hori-zontal, leading to an eight-celled stage, the upper four cellsbeing slightly smaller than the lower four. The succeedingdivisions are sub-regular, the cells near the upper pole tendingto remain smaller than their fellows at the opposite end.

Cleavage does not culminate in a solid mass of cells or morula,which MacBride found in the artificially fertilized eggs ofO p h i o t h r i x f r ag i l i s , but as he found in the naturallyfertilized eggs of this species, in a thick-walled blastula. Forma-tion of a blastocoele cavity begins as early as' the sixteen-celledstage as a space between the upper and lower tiers of cells. Asdevelopment progresses the blastocoele increases in size. Theblastula acquires cilia, and bursts the egg-membrane and be-comes free-swimming at the end of about twenty-four hours.Primary mesenchyme cells are budded off from one pole andoccupy a large portion of the blastocoele. The opposite poleof the egg-shaped larva is composed of a conical crest ofvacuolated cells which probably increase the floating powers ofthe larva. At the end of about thirty-six hours a slight in-pushing, which is the beginning of the archenteron, has appearedat the posterior pole and gastrulation has begun. As gastrula-tion progresses secondary mesenchyme cells are given off fromthe apex of the growing archenteron.

F o r m a t i o n of P r i m a r y B o d y - E o d s and Coelom.

The slit-like blastopore now assumes a more or less circular


form. The larva next assumes a helmet shape owing to theappearance of two lateral outgrowths which are the beginningsof the postero-lateral arms. Most of the mesenchyme cellstravel towards these projections, where they go to form thebasis of the calcareous rods which support the larval arms. Thespace between the archenteron and the outer ectoderm is aremnant of the segmentation-cavity or blastocoele, which Germanauthors have termed the primary body-cavity or archiocoele.

The coelom or secondary body-cavity next makes its appear-ance as a thin-walled sac from the tip of the archenteron. Infig. 9, PL 5, it is seen to be in communication with the archen-teric cavity. This connexion is cut off and the coelom becomesisolated as a thin-walled vesicle.

Meanwhile, a pit, the stomodaeal pit, lined by columnar cellsand having a prominent anterior lip, makes its appearance alittle in front of the middle of the ventral surface. By this timethe crest has entirely disappeared and the postero-lateral armshave elongated considerably. A transverse ridge, the 'longi-tudinal ciliated band', has appeared, and is becoming rapidlya horizontal loop. As the postero-lateral arms grow this loopextends into them, thus facilitating the swimming movementsof the larva.

The A l i m e n t a r y Canal .As the stomodaeum is being formed on the ventral side of

the larva, the gut, from which the coelom has detached itself,becomes by constriction divided into oesophagus, stomach, andintestine. The blastopore, the opening of the imagination toform the archenteron, persists as the anus. The stomodaeumnow joins the oesophagus, and the alimentary canal of the larvais complete. A thickened tract with masses of cilia projectsinto the oesophagus. This is the 'adoral ciliated band' and isformed partly by ectoderm and partly by endoderm cells.

A n t e r o - l a t e r a l Arms and the B o d y - E o d s .

About the fourth day two projections appear on the anteriorpole of the larva. These are the rudiments of the antero-lateralarms and are supported by branches given off from the main


skeletal rods which support the postero-lateral arms. At theposterior end of the larva, which is vacuolated, the primary rodsbranch into the anterior and posterior spikes, which in turndivide into' dorsal and ventral branches so that a sort ofinterlocking arrangement is seen.

F u r t h e r D e v e l o p m e n t of t he Ooelom.

We left the development of the coelom at the point where itis constricted off from the apex of the archenteron as a thin-walled vesicle. Having separated off into two portions thecoelom travels backwards on either side of the oesophagus toform the right and left coelomic sacs (fig. 10, PI. 5). Not in-frequently at first the right sac is slightly the larger. From theposterior ends of these sacs muscular fibres grow round theoesophagus. These are the 'constrictor muscles' which help thelarva in swallowing food. The left coelomic sac at first growsmore rapidly than the right one, and it sends out a hollowprocess dorsally which meets a short ectodermic ingrowth andforms a tube, the 'pore-canal' (fig. 11, PI. 5). Its externalopening is called the 'primary madreporic pore'. At about thistime the ' post-oral arms' are formed, as a result of which theventral surface of the larva becomes concave. The new armsreceive extensions from the primary body-rods arising very nearthe base of those supporting the antero-lateral arms.

Soon after the appearance of the post-oral arms the coelomicsacs undergo a marked change. Now, the left anterior coelom,which has a wider lumen than the right, sends out an extensionposteriorly, which is at first solid (fig. 12, PI. 5). This soonacquires a narrow lumen and separates off from the anteriorcoelom as the left posterior coelom, and travels backwards soas to be closely apposed to the left side of the stomach. Grave(1900) mentions that the coelom in Oph iu ra b r ev i soriginates as two distinct evaginations from the archenteron,of which the hinder forms the hydrocoele and left posteriorcoelom, and that the anterior evagination divides into rightand left anterior coeloms, the former later giving rise to the rightposterior coelom. This mistaken notion is attributable to faultyinterpretation, owing probably to stages being missed by Grave,


since Ophiura b rev i s has heavily yolked eggs and conse-quently undergoes very rapid development. Moreover, it neverdevelops a typical ophiopluteus larva. According to Masterman(1902), in the starfish Cribel la o c u l a t a somewhat similarresults occur, but the posterior evagination gives rise solelyto the left posterior or hypogastric coelom, all the other coelomicdivisions, including the hydrocoele, being formed from theanterior evagination of the archenteron. This in turn is a mis-taken observation as later work on Asteroids has shown. Thestage when the left posterior coelom is constricted off from theleft anterior coelom must have been missed by Mastermanowing to the extraordinary rapid growth of the larva of Cri-bel la o c u l a t a .

The right anterior coelom undergoes a similar change to theleft and gives rise to the right posterior coelom a little later,about the sixth or seventh day after fertilization. The rightanterior and the resultant posterior coelorns are smaller in sizethan their fellows on the left. Thus, in the case of Ophiocomanigra, as in A s t e r i a s , E c h i n u s , and O p h i o t h r i x , thepredominance of the left side of the larva, which plays such animportant part in the metamorphosis, over the right is main-tained.

E x t e r n a l Changes .

No further changes in the larva are noticeable for a few daysexcept the beginnings of the last pair of larval arms, the'postero-dorsal'. These receive branches from the skeletal rodssupporting the antero-lateral arms.

The H y d r o c o e l e s .

At about the sixteenth day after fertilization the posteriorend of the left anterior coelom becomes thickened. This is thebeginning of the left hydrocoele or the water-vascular rudiment(fig. 13, PI. 5). This thickening gradually grows in size andbecomes hollowed out and is constricted off from the anteriorcoelom (fig. 14, PI. 6). However, it does not separate fromthe latter completely but is connected with it by a narrowneck which is the rudiment of the 'stone-canal'. Bury (1899)

72 N. NABASIMHAMURTI

supposed that in Ophiuroids, unlike Echinoids, the left hydrocoeleoriginated from the left posterior coelom. He seems to havebeen led to this erroneous conclusion owing to the close proximityof the left posterior coelom to the hydrocoele. In later stagesof development the left posterior coelom almost touches theposterior portion of the hydrocoele, which grows considerablyin size, and therefore if an earlier stage is missed one is boundto conclude that the left posterior coelom gave rise to thehydrocoele.

E i g h t H y d r o c o e l e .

The posterior part of the right anterior coelom thickens inthe same way as its fellow on the left, a little later—about thetwentieth day (fig. 13, PI. 5). This is the right antirnere ofthe water-vascular rudiment. It corresponds to the righthydrocoele described by MacBride (1903) in E c h i n u s andby Gemmill (1914) in A s t e r i a s . MacBride mistook—inE c h i n u s (1903) and A s t e r i n a (1896)—another organ, thepericardial sac, which I have described in a previous paper(1932), for the right hydrocoele. This misinterpretation wascaused by the fact that the pericardial sac, like the righthydrocoele, arises from the posterior part of the right anteriorcoelom, the point of origin of the two organs being at differentlevels. In O p h i o t h r i x , MacBride correctly identified theright hydrocoele, although he found no traces of the pericardialsac in his sections. The right hydrocoele remains as a smallthickening with a narrow lumen. It does not constrict off fromthe right anterior coelom, but its cavity becomes entirely shutoff from that of the right anterior coelom though it lies closelyapposed to the latter (fig. 17, PI. 6).

When the larval arms have attained their full length, the lefthydrocoele grows anteriorwards, taking on a five-lobed form.The five lobes soon increase in length and become finger-shaped(fig. 18, PI. 6). The hydrocoele is so close to the wall of theoesophagus that it is very difficult to make out the anteriorcoelom in living larvae. Tor the sake of convenience the lobesof the hydrocoele are numbered in figs. 1 to 5, PI. 5, beginningwith the most anterior lobe.


The right hydrocoele does not undergo any such change.Muller (1846) has figured a larva of O p h i o t h r i x in whichthe right hydrocoele has become five-lobed. MacBride (1907)mentioned that occasionally in O p h i o t h r i x it assumes afive-lobed form, as is shown in his fig. 53. But in Ophio-coma, having cut and examined numerous sections, I havecome to the definite conclusion that the right hydrocoele remainsthroughout larval development as a thickening and does notassume a five-lobed form.

Left Hydrocoe le and Stone Canal .

Bury (1899) and MacBride (1907) disagreed with regard tothe opening of the stone canal into the left hydrocoele. Buryregards the opening as a fixed point for comparative purposesand, basing his statements on living Ophiuroid larvae, concludesthat the stone canal opens into the hydrocoele between lobes 4and 5. MacBride believed that this opening is situated betweenlobes 1 and 2. I n O p h i o c o m a (fig. 15, PI. 6), as in Ophio-t h r i x , the stone canal opens into the hydrocoele betweenlobes 1 and 2. The external opening of the pore canal is situatedopposite the interval between lobes 4 and 5. Since the stonecanal is in communication with the inner part of the ring, andthe lobes of the hydrocoele project from its outer surface, thepoint where the stone canal enters the hydrocoele can shiftwithout any difficulty. With regard to the remaining groupsof Echinoderms the evidence at present suggests that theopening occurs in a different inter-radius, and therefore furtherinvestigation is necessary before any broad conclusions can bedrawn.

The P e r i c a r d i a l Vesic le .

At about the thirtieth day after fertilization in serial trans-verse sections a mass of cells is seen lying near the dorsalsurface of the right anterior coelom, connected with thelatter by a thin protoplasmic strand. This mass of cells is therudiment of the pericardial vesicle. This is comparable tothe pulsating vesicle of the larvae of E c h i n u s mi l i a r i s andAs te r i a s r u b e n s , which arises as a thickening on the dorsal


surface of the right anterior coelom, and when constricted offfrom the latter is connected by a thin strand for a short time.Later it is isolated as a vesicle lying in the mid-dorsal line(fig. 16, PI. 6). In Ophiocoma this mass of cells soonacquires a cavity and moves over towards the left side, lyingmore or less in the mid-dorsal line. In Ech inus and As-t e r i a s larvae I was able (1932) to see this vesicle pulsate.Owing to the extreme opacity of the stomach of the larva ofOphiocoma at about this stage, I was not able to notice anypulsations of the pericardial sac. However, I was able to seepulsations in the just metamorphosed larva of Ophiocoma.Careful focusing through the tissues of the madreporic inter-radius revealed the presence of a rhythmically pulsating thin-walled cavity. The pulsations are regular and occur once in abouttwelve seconds. Therefore it is quite reasonable to assume thatin the larvae of Ophiuroids the pericardial vesicle pulsates in thesame way as in those of Echinoidea and Asteroidea.

S tomach and I n t e s t i n e s .The intestine is now loose and baggy, and the anus is kept

closed except while excreting. The stomach-wall at about thistime is quite thick. The cells lining the wall are composed ofa layer of rather closely-packed cylindrical cells. The outer endsof these cells are vacuolated. In sections of larvae preservedin osmic acid and Miiller's fluid, this vacuolated portion goesblack, owing probably to the presence of fat.

The Me tamorphos i s .It is rather difficult to fix a definite point which indicates

the beginning of metamorphosis in Ophiocoma n ig r a ,since the larvae, as in Ech inus and O p h i o t h r i x , remainswimming until the metamorphosis is complete. But in thelarvae of As te r ina g ibbosa and As te r i a s rubensmetamorphosis may be said to begin soon after the larva fixesitself. At fixation the hydrocoele is an open hoop in A s t e r i n a .In Ech inus the hydrocoele lobes are developed in the form ofa hoop but soon become ring-shaped. In Ophiocoma, as inO p h i o t h r i x , the lobes appear while the hydrocoele is stillstraight, and we may regard the beginning of metamorphosis


from the point at which the hydrocoele begins to grow roundthe oesophagus and assumes a hoop-like form.

The H y d r o c o e l e .

The larva has not been affected in any way with regard to itsexternal symmetry. The arms are quite long and healthy. Butchanges have been taking place in the hydrocoele. The lobesare long and prominent. The encircling of the oesophagus bythe hydrocoele in Ophiurid larvae has already been describedby Metschnikoff (1869), Bury (1889), Grave (1900), andMacBride (1907). In Ophiocoma the most anterior lobe ofthe hydrocoele first grows round the oesophagus over the mid-dorsal line towards the right side, closely followed by the secondand third lobes (fig. 7, PI. 5). The fourth and fifth lobes donot share in the movement as far as I could make out. NowMacBride (1907) mentions that in O p h i o t h r i x the posteriorend of the hydrocoele extends over to the right, parallel to theventral horn of the left posterior coelom. From my observationsI am able to make out that the fifth lobe does not completelypass over to the right side over the oesophagus but bends slightlyto the right (fig. 8, PL 5), to meet the first lobe, which on reach-ing the right side passes downwards ventrally to the anteriorcoelom and nearly joins the fifth.

Left P o s t e r i o r Coelom.

Meanwhile, the left posterior coelom has extended enormously.It has developed two horns which are termed ' dorsal and ventralhorns' (fig. 20, l'.p'.c'.,l".'p".c".). The dorsal horn extends for-wards along the left side of the larva, outside the hydrocoele lobes,to reach up to about the level of the third lobe. The ventral horn,which is in communication with the dorsal horn at its origin, moveshorizontally across the oesophagus to the right side and curvesupwards. Eventually at a later stage the two horns fuse, forminga ring outside and parallel to the hydrocoele-ring.

From the inner wall of the left posterior coelom four exten-sions grow out, alternating between the lobes 2 and 3, 3 and 4, 4and 5, and 5 and 1 of the hydrocoele (fig. 20, PL 6). Theseoutgrowths are the rudiments of the 'Perihaemal' system of

76 N. NARASIMHAMUETI

cavities. The muscles and the coelomic nerves (Lange's nerves)of the adult Ophiuroid develop later from their walls. Theseoutgrowths are at first thick-walled, and later they acquirea lumen in their middle. At first they are in communicationwith the left posterior coelom, but are soon entirely cut off andextend as U-shaped canals within the interradii. There stillremains the fifth perihaemal diverticulum between lobes 1 and 2of the hydrocoele to be accounted for. MacBride, in A s t e r i n a(1896), as well as O p h i o t h r i x , believed that it originatesfrom the left anterior coelom as an outgrowth. Gemmill (1914),in A s t e r i a s rubens , believed it arises from the dorsal hornof the left posterior coelom. From my sections of Ophiocomait seems to arise from the dorsal horn of the left posteriorcoelom (fig. 20, PL 6), close to the ampulla of the stone canal.The subsequent closure of the perihaemal stalk and the forma-tion of a ring by the horns of the left posterior coelom make itappear as if the diverticulum originated from the left anteriorcoelom.

By this time the rudiments of the adult arms have appeared.These arise as conical thickenings of the outer wall of the leftposterior coelom. Of these outgrowths three are dorsal and twoventral. These thickenings, when first formed, are solid butsoon acquire a cavity in them. This cavity is the forerunner ofthe 'dorsal coelomic canal' of the adult Ophiuroid. A greatmany of the outer cells of these thickenings go to form thecalcareous structures which ensheath the adult. Unfortunately,owing to the methods of preservation adopted, the plates wereeither dissolved out or destroyed.

E x t e r n a l Changes .Now the first three lobes of the hydrocoele have extended

over to the right and downwards so far that the water-vascularring is almost complete. Soon the external features of the larvaare affected. The left side of the oesophageal wall has thickenedconsiderably. The antero-lateral arms have shortened in size andbecome stumpy. The whole of the anterior pole of the larvais bent over to the right. The madreporic pore and the leftantero-lateral arm have inclined to the right half of the body.


The left anterior coelom and the stone canal have shared in themovement and now lie approximately in the mid-dorsal line.The adoral ciliated band has thickened considerably and thethin-walled portion of the oesophagus has been completelypushed over to the right and has diminished in size. It appearsas a mere slit encircled by the lobes of the hydrocoele. In short,we may attribute these changes to the tendency of the mouthto move towards the left. Meanwhile, the intestine has becomediminished in size and is slanting to the right. Its cells havebecome vacuolated and are obviously in the first stage ofdegeneration.

The P r i m a r y Ten t ac l e s and E p i n e u r a l Cana l s .

In the next two or three days the tips of the hydrocoele lobesproject into the outer part of the stomadaeum as 'primarytentacles'. Inter-radially outgrowths of ectoderm appearalternating with these tentacles. From these ectodermal ridgesflaps grow out on either side. These flaps meet and fuse together toform the ' epineural canals' (fig. 25, PI. 6) or the closed ' ambu-lacral grooves' characteristic of Ophiuroids. In this respect theOphiuroidea resemble Echinoidea, for the epineural ridges ofE c h i n u s e s c u l e n t u s arise in a similar manner accordingto MacBride's (1903) observations.

The next stage in the metamorphosis may be said to havebegun with the completion of the hydrocoele-ring by the fusionof its dorsal and ventral ends.

E x t e r n a l Changes .Now the left antero-lateral arm is so far pushed over to the

right side that it is in close contact with its fellow on the right(fig. 8, PL 5). This is brought about partly by the preponderantgrowth of the organs on the left side, mainly the encircling ofthe oesophagus by the hydrocoele, and partly by the shrinkageof the intervening space between the two most anterior armsof the larva. This shrinkage reminds one of the atrophy of thepre-oral lobe in Asterids, especially A s t e r i n a g i b b o s a .Therefore we may venture to suggest that the forepart of theOphiuroid larva is homologous with the pre-oral lobe of Asterids.

78 N. NARASIMHAMUETI

P a i r e d T e n t a c l e s and Nerve -Ce l l s .Now the rudiments of the tentacles begin to appear in the

hydrocoele lobes. Each lobe has given rise to three pairs oflateral lobes which are the beginnings of the paired tentacles.The original middle lobe gives rise to the terminal or azygoustentacle on each arm. New tentacles arise by buddings fromthe main tentacle. The perihaemal rudiments have separatedfrom the left posterior coelom and their cavities have extendedin a V-shaped manner, one limb of V extending up each arm.From these extensions, fine lightly-staining wavy fibres withconnecting sets of nuclei appear. These are the motor ganglion-cells of the 'coelomic nervous system' (fig. 24, PL 6). At thesame time other nerve-cells arise from the ectodermal coveringof the tentacles. These cells go to form the 'ectodermal nervoussystem' (fig. 25, PI. 6) of the adult Ophiocoma. In sectionsother large cells with few nuclei are seen to arise from the cavity,and similar cellular growths appear from the dorsal coelomiccanal. These cells in all probability are the beginnings of theadult 'intervertebral muscles'.

E i g h t H y d r o c o e l e and P e r i c a r d i a l Ves ic le .

We must now pause to consider the fate of the right hydro-coele. MaeBride (1907) has figured a stage where it has assumeda five-lobed form.1 Though I have cut a good many sections,I have not been able to get a similar stage. Its normal form inall my sections is as a small thickening of the posterior end ofthe right anterior coelomic sac. As far as I can make out, theright hydrocoele persists only up to the end of metamorphosisand does not exist in the imago of Ophiocoma. When thehydrocoele forms a complete ring round the oesophagus theright hydrocoele is seen to be a flattened-out structure, and inlater stages it thins out and disappears. Therefore, the sugges-tion that this organ, which serves no useful purpose in the adultmechanism of the Ophiuroid, degenerates as metamorphosisprogresses, seems to be quite plausible. MaeBride (1907) furtherstates that in the metamorphosing stages of O p h i o t h r i x

1 I am satisfied that this figure, which appeared in my paper of 1907,is based on a misinterpretation.—E. W. MaeBride.


f rag i l i s a projection of the inner wall of this organ is notice-able, similar to that which gives it a crescentic form in the larvaof As te r ina gibbosa (1896). MacBride seems to havemistaken here, as in the case of Ech inus and A s t e r i n a ,the pericardial sac for the persistent right hydrocoele. Theinpushing of the ventral wall of this sac goes to form the' heart' of Ophiuroids. I have already mentioned that the peri-cardial sac arises in Ophiocoma from the dorsal surface ofthe right anterior coelom at a higher level than where the righthydrocoele is given off. It arises in a similar way in Echinoidsand Asteroids. This sac persists in the adult Ophiocoma asa crescentic vesicle lying beside the stone canal immediatelybeneath the madreporic plate (fig. 23, PI. 6). What MacBridecalls sinus B in his description and sketches of Amphiu raa q u a m a t a (1892) corresponds to this vesicle. In the justmetamorphosed larva of Ophiocoma I have been able tosee faint pulsations in this portion. Careful focusing throughthe madreporic inter-radius, with the help of powerful illumina-tion, revealed faint pulsation in this region. Contractions couldbe made out with ease from the aboral side. The pulsationswere rather regular, occurring once in every ten or twelveseconds. There could be no doubt that this thin-walled cres-centic cavity is homologous with the pulsating pericardialvesicle I described in Ech inus mi l i a r i s and As te r i a srub ens (1932), and which I regarded as comparable to thepericardium and heart of Ba l anog los sus . The tissue under-neath fills the invaginated heart of the animal.

The right anterior coelom has by now disappeared and theleft anterior coelom has enlarged to form the ' ampulla' of thestone canal (fig. 23, PL 6). The ampulla is the homologue ofthe axial sinus of As te r ina gibbosa and Ech inusmi l i a r i s .

The Diges t ive Canal .

Changes have been taking place in the alimentary canal. Thedistal portion of the stomodaeum has disappeared owing to itsflattening out and its constituent cells being merged in theectoderm, and the oesophagus is thick walled. This wall is


derived chiefly from the left half of the adoral ciliated band.The intestine is reduced to a small solid rod of vacuolated cells.The stomach appears in sections as a solid mass. Later it is seento send out five projections alternating with the arms (fig. 25,PI. 6). The adult oesophagus is formed from the inner portionof the larval oesophagus, and the peristomial membrane of theadult Ophiuroid is derived from the outer half which has beenflattened horizontally and uncovered owing to the shrinkageof the forepart of the larva. In this respect Ophiuroidea re-semble the Holothuroidea, where the buccal membrane and theadult oesophagus are formed in the above manner; but in Echi-noids and Asteroids the adult mouth arises as a result of themeeting of ectoderm and endoderm on the left side of the larva.

F i n a l S tage of Metamorphos is . .

The last stage in the metamorphosis consists in the disappear-ance of all larval arms. But the postero-lateral arms persistfor a long time, and are the last to be shed. The adult arms haveincreased in length and have become apposed to the lobes ofthe hydrocoele by the shrinkage of the ectoderm connectingthese two structures. The form of the Ophiuroid, which waspentagonal in the previous stage, is now distinctly five-rayed.

Now, there arises from the left posterior coelom a narrowcavity which insinuates itself between the left posterior coelomand the oesophagus. This is the' peri-oral coelom' correspondingto that found in Asteroids. This space apparently persists inthe adult Ophiuroid as a functionless vestige, but in theAsteriod the dividing wall between the left posterior coelomand the peri-oral coelom breaks down and gives rise to theten retractor muscles of the stomach. This may be explainedby the fact that the stomach of the Ophiuroid is not eversiblelike that of the Asteroid.

Meanwhile, an interesting change has been taking place inthe wall of the left posterior coelom overlying the stone canal.A cluster of darkly-staining long nuclei have made their appear-ance. These are the 'primitive germ-cells' (gen., fig. 4, PI. 5).These are at first in the form of a single layer of cells, but laterproject into the concavity of the pericardial vesicle as solid

DEVELOPMENT OF OPHI0C0MA 81

nodule. This is the rudiment of the ' genital rachis' of MacBridewhich eventually gives rise to the genital organs by budding.

By this time the oral spines which develop the so-called teethhave already appeared. The ciliated epithelium enveloping thepostero-lateral arm degenerates and the animal reaches thebottom and begins to walk on its tube feet. The postero-lateralarms shrink and finally disappear, exposing the spines. Thenaked spines break off and the young brittle-star emerges.

GENERAL CONCLUSIONS.

We shall now summarize the facts above related and see whatconclusions can be drawn from them as to the resemblancesand differences in the developmental history of the variousgroups of Echinoderms.

The development of the coelom forms one of the most im-portant links in connecting up the different groups of Echino-derms. In all cases the coelom arises as a single bi-lobedevagination from the apex of the archenteron. In A n t e d o nrosaoea , the only crinoid whose development has been studied,the whole archenteron becomes converted into the coelom whichthen divides into anterior and posterior portions. The futuregut of the larva arises as a pair of outgrowths from the anteriorportion. This late appearance of the gut can be accounted forby the fact that it serves no function in the embryo. In theHolothurians the single outgrowth first divides into anteriorand posterior outgrowths. The anterior gives rise to the anteriorcoelom and the left hydrocoele, while the posterior portiondivides into right and left halves. In Asteroids, Echinoids, andOphiuroids, the coelom first divides into right and left halvesand these subsequently give rise to the posterior portions andthe hydrocoele. In the star-fishes A s t e r i n a g ibbosa andCribe l la o c u l a t a the coelomic rudiment does not divideinto right and left halves as is the case in A s t e r i a s rubens ,where the single anterior coelom is formed in the pre-oral lobeby the union of the right and left halves. This peculiarity maybe explained by the fact that certain stages are skipped overowing to the shortening of the development in A s t e r i n a and

NO. 301 G

82 N. NAEASIMHAMUETI

Cr ibe l l a . Again, in the larva of Ophiocoma, as well asO p h i o t h r i x , the right anterior coelom invariably gives riseto a right hydrocoele. In Asteroids and Echinoids this occursonly as a variation. From the dorsal surface of the right anteriorcoelom arises a contractile pericardial vesicle in all the threegroups. The pericardial vesicle is absent in Holothuroids owingprobably to the extreme contractility of the body-wall, and itspresence has not been established in a crinoid.

In Echinoids the hydrocoele arises as a disk. Later it becomesring-shaped. Before the ring shape is complete the hydrocoelehas the form of a hoop, recalling its form in A s t e r i a s ,O p h i o t h r i x , and Oph iocoma . The left posterior coelomgives rise to the dorsal and ventral horns which fuse and forma ring encircling the water vascular ring in Oph iocoma , anda similar thing happens in A s t e r i a s . The left posteriorcoelom in Ophiocoma sends out five pocket-shaped pro-jections. These evaginations are the homologues of the peri-haemal spaces of the Asteroid and Echinoid larvae. MacBridementions that one of these pockets arises from the left anteriorcoelom in A s t e r i n a as well as O p h i o t h r i x ; but in Ophio-coma there is no doubt whatever that all the five originatefrom the left posterior coelom, and this recalls their similarorigin in A s t e r i a s r u b e n s (Gemmill, 1914).

The atrophy of the forehead of the larva of Ophiocoma iscomparable to the degeneration of the pre-oral lobe of theAsteroids. The points in which the development of Ophio-coma differs from that of an Asteroid are: (1) the coelomiccavities are in general smaller and their walls thicker; (2) thefixed stage is absent; and (3) the larval mouth persists in theadult.

In the absence of a fixed stage, and in the consequent reten-tion of the locornotor organs till the tube feet become functional,the larvae of Ophiuroidea resemble the Echinoids. This absenceof a fixed stage, which furnishes us with the transitional condi-tion between a bilaterally symmetrical animal swimmingfreely, and a radially symmetrical animal creeping at thebottom of the sea, may be explained as the result of a tendencyto reduce the number of larval stages.


The relatively small size of the cavities and the thickness ofthe resultant tissues is a general feature of the Ophiuroids.

The Ophiopluteus differs from the Asteroid and Echinoidlarvae in the retention of the larval mouth. The Brachiolariaand the Echinopluteus form a new mouth. In Holothuroideaand Crinoidea the mouth persists. The larval stomodaeum ofA n t e d o n rosacea does not open into the gut; but still,from its position and structure, corresponds to the stomodaeumof other Echinoderm larvae. When the larva fixes itself thestomodaeum shifts posteriorly and opens out, forming the vesti-bule from which the tentacles project. In the Holothuroids,according to Metschnikoff's (1869) account, the terminal ten-tacles when they make their appearance first project into the'atrium', as the stomodaeum is termed by him. In Ophio-t h r i x and Oph iocoma , owing to the shrinkage of the fore-head, the stomodaeum is laid bare and the primary tentaclesof the hydrocoele project into it. Now in the Echinopluteus, thetube feet protrude into the amniotic cavity. Therefore thestomodaeum, into which the primary tentacles project inOphiuroids, is comparable to the amniotic cavity of Echinoids.We may interpret the amniotic cavity as a part of the stomo-daeum which is formed separately and then shifted to the leftside to occupy the adult position to protect the first-formedtentacles, while the other retains its position in the middle tocomplete the larval alimentary system.

SUMMARY OF NEW POINTS.

1. Cleavage results in a thick-walled blastula and not amorula as in the artificially fertilized eggs of Ophiothrix.

2. The coelomic divisions are thick-walled and their cavitiesare much smaller than in Ophiothrix.

3. A right hydrocoele arises as a thickening of the rightanterior coelom in all the larvae, but it does not assume a five-lobed form in any case.

4. The right hydrocoele does not persist in the adult, butdegenerates as metamorphosis progresses.

5. A pericardial vesicle arises from the dorsal surface of theG2


right anterior coelom as in Echinoids and Asteroids. It persistsin the adult as a thin-walled vesicle lying beside the pore canal.

6. A heart is formed by the invagination of the ventral wallof this vesicle. Pulsations occur once in about twelve secondsin the ventral wall of the vesicle, in the just metamorphosedforms.

7. During metamorphosis the fifth lobe of the left hydrocoeledoes not move across the oesophagus to the larval right as inO p h i o t h r i x , but it bends slightly to the right to meet thefirst lobe which travels towards it after passing round theoesophagus.

8. All the perihaemal spaces arise as pocket-shaped evagina-tions from the left posterior coelom.

9. The stomach ofOphiocoma is peculiar. The outer endsof the cylindrical cells are vacuolated and stain deep black inosmic acid and Muller's fluid, owing to the presence of fat inthem.

10. In post-larval stages the stomach at first appears as asolid mass, but later sends out five projections alternating withthe arms.

In conclusion, I have pleasure in expressing my indebtednessto Professor E. W. MacBride, F.R.S., under whose guidancethe work was carried out, for his constant advice and construc-tive criticism which have gone a long way in improving thispaper. I must also thank Dr. E. J. Allen, Director of the MarineBiological Laboratory at Plymouth, for providing me with thematerial required, and giving me facilities to carry out thoseparts of the work which were done in the Plymouth laboratories.I must also thank my friend Mr. J. K. Deuskar to whose skillI owe the excellent figures in this paper.

REFERENCES.

Agassiz (1883).—"Embryologioal Monographs. II. Eohinodermata",'Mem. Mus. Comp. Zool. Harvard', vol. ix, pt. 2.

Apostolides (1882).—"Anatomie et developpement des Ophiures", 'Arch.Zool. Exp.', vol. x.

Balfour, F. M. (1881).—'Text-Book of Comparative Embryology', vol. 1.Bury, H. (1888).—"The Early stages in development of Antedon rosacea",

'Phil. Trans. Roy. Soc.', vol. 150.


Bury, H. (1889).—" Studies in Embryology of Echinoderms " , ' Quart. Journ.Micr. Sci.', vol. 29.

(1895).—"Metamorphosis of Echinoderms", ibid., vol. 38.Carpenter, P. (1887).—"On Development of Apical plates in Amphiura

Squamata", ibid., vol. 27.Driescb, H. (1895).—"Zur Analysis der Potenzen embryonaler Organ-

zellen", 'Arch. Entwickelungsmechanik', vol. ii.Eewkes, J. W. (1886).—"Preliminary Observations on Development of

Ophiopholis and Echinarchnis", 'Bull. Mus. Comp. Zool. Harvard',vol. 12.

(1887).—"Development of the calcareous plates of AmphiuraSquamata", ibid., vol. xiii.

Gemmill, J. I \ (1911).—"Notes on the adult anatomy of Solaster endica",'Proc. Roy. Phys. Soc. Edinburgh', vol. 18.

(1912).^"Development of the Starfish Solaster Endica", 'Trans.Zool. Soc. London', pt. 3.

(1914).—"Development and adult structure of the Starfish, AsteriasRubens", 'Phil. Trans. Roy. Soc. London', vol. 205.

(1915).—'Quart. Journ. Micr. Sci.', vol. 61, pp. 27-50.(1915).—"Double Hydrocoele in Development and Metamorphosis of

larva of Asterias rubens", ibid., vol. 61, pp. 51-80.(1918).—"Rhythmic pulsation in the Madreporic Vesicle of Young

Ophiuroids", ibid., vol. 10, pp. 239-78.Goto, S. (1898).—"Metamorphosis of Echinoderms with special reference

to fate of Body Cavities", 'Journ. Coll. Sci. Imp. Univ. Tokyo', vol. 10.Grave, C. (1900).—"Ophiura brevispina", 'Mem. Biol. Lab. Johns Hopkins

Univ.', vol. 4, no. 5.Kowalevsky, A. (1867).—"Entwickelungsgeschichte des Amphioxus

lanceolatus", 'Mem. Acad. Imp. des Sc. S.-Petersbourg', vol. xi, series 7.Krohn (1851).—"Entwickelung eines lebendig-gebarenden Ophiura",

'Arch. Anat. u. Physiol.'Ludwig, H. (1881).—"Zur Entwickelungsgeschichte des Ophiurenske-

lettes", 'Zeit. fur Wiss. Zool.', vol. xxxvi.MacBride, E. W. (1892).—"Development of genital organs, ovoid gland,

axial and aboral sinuses in Amphiura squamata", 'Quart. Journ. Micr.Sci.', vol. 34.

(1896).—"Development of Asterina Gibbosa", ibid., vol. 38.(1903).—"Development of Echinus esculentus", 'Phil. Trans. Roy.

Soc.', vol. cxcv.(1907).—"Development of Ophiothrix fragilis", 'Quart. Journ.

Micr. Sci.', vol. li.(1912).—'Text-Book of Embryology, Invertebrates.'(1918).—"Development of Echinocardium cordatum" 'Quart.

Journ. Micr. Sci.', vol. 63.


Masterman, A. T. (1902).—"Early Development of Cribella Oculata",'Trans. Roy. Soo. Edin.', vol. xi, p. 2.

Metschinikofi, E. (1869).—" Entwickelung der Echinodermen und Nemer-tinen", 'Mem. Acad. Imp. des Sc. S.-Petersbourg', vol. xiv, series 7.

Mortensen (1900).—"Die Eohinodermenlarven der Plankton-Expedition",'Ergeb. der Plankton-Exp.', Bd. 2, J.

(1900).—"Die Echinodermenlarven", 'Nordisches Plankton', ix.Miiller (1846).—"Ueber einige neue Thierformen der Nordsee", 'Arch.

Anat. u. Physiol.'(1850).—"Larven und Metamorphose der Holothurien und Asterien",

'Konig. Akad. Wiss. zu Berlin.'(1851).—"Ophiurenlarven des Adriatisohen Meeres", ibid.(1852).—"Ueber den allgemeinen Plan in der Entwioklung der

Echinodermen", ibid.Narasimhamurti, N. (1932).—"Development and Function of Heart and

Pericardium in Echinodermata", 'Proc. Roy. Soc'Russo, A. (1891).—"Embryologia dell'amphiura squamata (Sars)", 'Atti

del. R. Aoademia del Sc. fis. e mat. di Napoli', vol. 5, series 2.Schulze, M. (1851).—"Entwicklung von Ophiolepis Squamata", 'Arch.

Anat. u. Physiol.'Selenka; E. (1883).—"Keimblatter der Echinodermen", 'Studien liber

Entwicklung der Thieren', Weisbaden.Woltereck, R. (1904).—"Wurm-Kopf, Wurm-Rumpf und Trochophora",

'Zool. Anzeiger', Bd. 28, no. 849.Ziegler, H. E. (1896).—"Entwicklungsgesehichte der Echinodermen",

'Verh. der deutschen Zool. Ges.'

EXPLANATION OF PLATES 5 AND 6.LETTERING.

al., antero-lateral arm; b.t., buccal tentacle; cil.ad., adoral ciliatedband; dl.bn., longitudinal ciliated band; coe., coelomic rudiment; ep.c,epineural canal; gen., primitive germ-cells; int., intestine; l.a.c, left anteriorcoelom; l.hy., left hydrocoele; l.p.c, left posterior coelora; l".p".c"., dorsalhorn of the left posterior coelom; V.p'.c'., ventral horn of the left posteriorcoelom; mes., mesenchyme; m.p., primary madreporic pore; mus.f., muscle-fibres; ».c, ectodermic nerve-cells; oes., oesophagus; p.c, pore canal;p.d., postero dorsal arm; ph., perihaemal space; ph. 1 2, 2 3, 3 4, 4 5, 5 1,perihaemal rudiments originating between lobes 1 and 2, 2 and 3, 3 and 4,4 and 5, and 5 and 1 of the hydrocoele; pi., postero-lateral arm; po., post-oral arm; r.a.c, right anterior coelom; r.hy., right hydrocoele; r.p.c,right posterior coelom; sk., larval skeletal rod; or so-called teeth of theadult; st., stomach; st.c, stone canal; stom., stomodaeum; vac., vacuolatedectoderm at posterior end of larva; vac.c, vacuolated outer ends of thecvlindrical cells of the stomach.


PLATE 5.

Kg. 1.—Optical section of gastrula 36 hours old. The vacuolated crestis well developed.

Fig. 2.—Ventral view of larva 48 hours old.Pig. 3.—Ventral view of larva 4 to 5 days old. Right and left undivided

ooelomic vesicles and the calcined rods supporting the arms are shown.Fig. 4.—Dorsal view of larva about 6 days old. Anterior and posterior

halves of the left coelom still connected. Bight posterior coelom is alsodistinguishable.

Fig. 5.—Ventral view of larva about 14 days old. Postero-dorsal armshave just begun to develop and right and left hydrocoeles have appeared.

Fig. 6.—Ventral view of larva, about 20 days old. All the arms haveattained their full length. The left hydrocoele has grown forward to aboutthe level of the tip of the left anterior coelom and is five-lobed.

FIG. 7.—Ventral view of larva about 25 days old. Lobes 1, 2, and 3 ofthe hydrocoele are moving across the oesophagus.

Fig. 8.—Ventral view of larva 30 to 35 days old. Lobes 1, 2, and 3 ofthe hydrocoele have passed over the oesophagus and travelled downwardsso as to be almost in touch with lobe 5 which has slightly inclined towardsthe left.

Fig. 9.—Longitudinal frontal section of larva 3 days old. Notice theorigin of the eoelom and the mesoderm cells that go to form the calcareousrods supporting the arms.

Fig. 10.—Longitudinal frontal section of larva about 3J days old. Rightand left anterior coeloms can be made out.

Fig. 11.—Transverse section of larva 4 days old. Ectodermie invagina-tion to form the madreporic pore is shown.

Fig. 12.—Longitudinal frontal section of larva 5 to 6 days old. Out-growth from the right anterior coelom or the right posterior coelom is stillconnected with the former. The left posterior coelom has already separatedfrom the left anterior coelom.

Fig. 13.—Longitudinal frontal section of larva 12 to 14 days old. Theleft hydrocoele is getting constricted off the left anterior coelom while therudiment of the right hydrocoele has appeared.

PLATE 6.

Fig. 14.—Longitudinal section of larva about the same age as foregoingbut slantingly cut. The rudiment of the left and right hydrocoeles are seen.

Fig. 15.—Longitudinal section through a larva about 25 days old showingthe opening of the stone canal into the left hydrocoele between lobes 1 and 2and the opening of the pore-canal to the exterior.

Fig. 16.—Transverse section of larva about 25 days old, showing theleft hydrocoele, left anterior coelom, and the pericardial vesicle in then-relative positions.

FIG. 17.—Longitudinal frontal section of larva about 30 days old. The

88 N. NABASIMHAMURTI

lobes of the left hydrocoele are finger-like. The right hydrocoele is justa thickening at the posterior end of the right anterior coelom.

Fig. IS.—Longitudinal frontal section of larva about the same age asthat figured in 17. The lobes of the left hydrocoele are long and prominent.The right hydrocoele is also seen.

Fig. 19.—Longitudinal frontal section of larva about 35 days old. Thehydrocoele lobes have encircled the oesophagus. Ventral horn of the leftposterior coelom is prominent.

Fig. 20.—Another longitudinal frontal section of the same larva as thatshown in fig. 19. The dorsal and ventral horns of the left posteriorcoelom are clearly marked out. Perihaemal spaces 1, 2; 2, 3; 4, 5 areshown.

Fig. 21.—Longitudinal section of larva, to same age as the foregoing.Formation of perihaemal space 4, 5 is shown.

Fig. 22.—Longitudinal section of larva rather slantingly cut, showingperihaemal rudiment 2, 3.

Fig. 23.—Transverse section (highly magnified) showing the relativepositions of the pore canal, stone canal, ampulla, and the pericardial vesiclein the just metamorphosed larva.

Fig. 24.—Longitudinal section of the just metamorphosed larva. Stonecanal, ampulla, pericardial vesicle, primitive germ-cells, &c, are seen.

Fig. 25.—Transverse section of the just metamorphosed larva showingthe nerve-cells and muscle-fibres and epineural sacs.

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the development of ophiocoma nigra

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