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internal ciliated epithelium. Studding the perisome are numerous spines, attached to the ossicula on the dorsal surface and to those bordering the ambulacral grooves; sometimes also there are tufts of bristles, the paxilla. The pedicellariæ are attached to the perisome and spines, and are either sessile or provided with short foot-stalks. Except in one group, they have two blades only, which are moved by divaricator and adductor muscles.

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The lower or oral surface of the star-fish with the ambulacra corresponds to the ambulacral, the aboral or antambulacral surface to the interambulacral areas of the echinus. The deep ambulacral grooves which occupy the middle of the lower face of each ray are formed each by a series of plates, the vertebral ossicles, articulated to one another by their inner opposed ends, and united by their avertebral ossicles; b. adambulacral ossicles; lower or outer ends to rows of plates, the adambulacral margins of the grooves, and are one or more series of marginal outer ends of the innermost pairs unite round the mouth to form five crests, which bear spines and pedicellariæ. On the aboral surface of the body are the tergal plates (fig. 13). Transverse muscular fibres unite the lateral halves of the arm-segments; similar fibres supply the floor of the ambulacral groove; besides

FIG. 12.-Section of ray of Astropecten aurantiacus.

c, d, marginal ossicles; e, paxillæ. (After Gaudry.)

ossicles, which form the themselves succeeded by ossicles (fig. 12). The of ambulacral ossicles

these there are intervertebral FIG. 13.-Tergal skeleton of and interambulacral longitudinal Asterias rubens. muscles. The ambulacral grooves a connecting pieces; b, spineare nearly filled with the tube-feet

bearing plates. (After Gaudry.)

or pedicels, which have a nervous external and muscular internal layer, are usually cylindrical in form and furnished with terminal sucking-disks, and communicate by ducts passing through the ambulacral pores with vesicles lying above the ambulacral ossicles and opening into the ambulacral canal of the ray. In the common star-fish, Asterias (Asteracanthion) rubens the pores form a zig-zag line on each side of the ambulacral groove, and the pedicels passing through them thus come to be fourranked (fig. 14). They are formed by notches or semi

b

FIG. 14.-Asterias rubens.

a, 4-ranked pedicels; b, end of pedicel, magnified.

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gullet into the FIG. 15.-Ossicles of ambulacral groove of Asterias stomach. The

rubens, viewed from above. a, pore for pedicel.

stomach in most star-fishes is produced into five sacculated prolongations (cardiac sacs); above these it contracts, but again widens to form the pyloric sac; this gives rise to five tubes, which open out in each ray into a pair of parallel diverticula having numerous cæcal dilatations, and connected by a mesentery with the antambulacral perisome. The pyloric sac in most cases leads into a short intestine terminating in an anus situated in the left posterior interradial space. In Astropecten, Ctenodiscus, and Luidia there is no anus. The madreporic tubercle is situated dorsally in the body disk, near one of the interradial angles; it is oval or slightly pentagonal in form, and the surface is marked with undulating grooves, and is finely perforated (fig. 16). In some genera

*

FIG. 16.-Antambulacral surface of Asterias rubens.
a, madreporite; a', the same magnified; b, anus.

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(Ophidiaster, Echinaster) there are several interradially placed madreporic tubercles. The doubly involuted madreporic canal is invested by the peritoneal membrane, which incloses a sinus, or heart," as it has been termed; it passes downwards into a pentagonal circum-oral ring which gives off the five radial canals occupying the uppermost part of the ambulacral grooves. The circum-oral ring may or may not possess Polian vesicles. A dorsal or aboral ring has been described as communicating with the "heart," and sending off interradial branches to the genital glands, the products of which, in the case of starfishes devoid of external genital apertures, it has been supposed they are the means of removing. The genital glands are racemose masses placed interradially in pairs; their processes sometimes extend a considerable distance into the arms. The nervous system consists mainly of a circular canal around the gullet, with five ambulacral trunks opening into it at their inner ends. The ambulacral neural trunk in each ray underlies a strong band of transverse fibres, by which it is separated from the ambulacral canal above. At the extremity of the ray the nerve terminates in an eye and its tentacle. The eyes are small processes of the ectoderm, having a convex surface or cornea containing a large number of simple, conical, pigmented ocelli. In the peritoneal cavity and ambulacral vessels is a watery fluid containing corpuscles. Respira

pores incised one on the distal and the other on the oral tion appears to be effected by means of water supplied

VII.

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to the interior by fine closed tubuli or dermal branchia | perisome. The dermal skeleton of the arms is constituted between the plates of the perisome. In Solaster water can usually of a ventral or superambulacral row of plates, a enter the body-cavity by the interbrachial cribriform plates

through which the genital ducts pass.

Among the Asteridea several modes of development have been observed. In some species reproduction appears at times to be effected by division of the rays. The species Pteraster militaris hatches its young in a special pouch on the dorsal surface. The larva on leaving its egg is oval, but subsequently assumes a pentagonal form, and the provisional mouth comes to be placed at one of the body angles. The central mouth and stomach afterwards developed open into each other at the time that the young star-fish leaves the maternal pouch. In other cases the breeding-chamber may be formed by the bringing together of the bases of the rays, and the ciliated embryo develops at its anterior end club-shaped tubercles, by which it can attach itself to the breeding-chamber or to submarine objects. Until these processes appear the breeding. chamber remains closed. In general, the larva of the Asteridea begins life as a lobed and ciliated pseud-embryo, a common form of which is the Bipinnaria. Another form, the Brachiolaria, is distinguished principally by three tuberculated processes at the anterior end of the body. The ambulacral vessels of the adult are developed in the pseud-embryo from a portion of one of the diverticula of the stomach in which originate the peritoneal cavity and the whole or great portion of the mesodermic structures. The Asteridea are classed by M. Edmond Perrier as follows:DIVISION I. Pedicellaria pedunculated; pedicels (except in Labidiaster and Pedicellaster) quadriserial. ASTERIIDE. Ex. Asterias (Asteracanthion), Heliaster, Calvasterias, Anasterias, Labidiaster, Pedicellaster. DIVISION II.-Pedicellaria sessile pedicels ordinarily biserial. i. Dorsal skeleton reticulate.

Cribrella.

EOHINASTERIDE. Ex. Acanthaster, Solaster, Echinaster, i. Dorsal skeleton of longitudinal series of rounded or quad. rangular ossicles; integument generally granulated. LINCKIADE. Ex. Ophidiaster, Linckia, Scytaster. ii. Skeleton, at least of lower surface, of tesselated ossicles; dorsal and ventral marginal plates very distinct. GONIASTERIDE. Ex. Pentagonaster, Goniodiscus, Goni. aster, Culcita, Asterodiscus, Choriaster. iv. Skeletal ossicles imbricated; with spines on the free border, or rounded and completely covered with small spines. ASTERINIDE. Ex. Palmipes, Asterina, Nepanthia. v. Skeleton of paxillæ. ASTROPECTINIDE. Ex. Chataster, Luidia, Astropecten, Archaster, Ctenodiscus.

vi. Dermal investment supported by spines radiating from the prominent skeletal ossicles. PTERASTERIDE Ex. Pteraster.

vii. Arms long, straight, distinct from disk, with minute spines

on dorsal surface.

BRISINGIDE. Ex. Brisinga.

f

Fra. 18.-Ophiopholis bellis, upper surface.

dorsal median or antambulacral row, and two side rows of spinous imbricated plates. More rarely the perisome of the arms is leathery in consistence, and bears small plates, double line of pores. of which the ventral are the largest, and perforated with a The internal axial skeleton is

formed by the vertebral or axial ossicles (fig. 19), the right and left halves of which are united by a longitudinal suture. The axial ossicles are articulated to one another by means of peg-and-socket joints. On the lower surface of each, corresponding to a depression on its dorsal surface, there is a groove for the passage of a radial ambulacral The innermost of vessel and a nerve.

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the axial ossicles is in two articulated halves, and the neighbouring halves of every two arms are connected with a couple of interambulacral pieces, with the inner edges of which is articulated a single ossicle, the torus angularis. The last-mentioned bears the papilla angulares, and beneath these the pala angulares, which are short flat processes, moved by muscles, and serving as teeth (see fig. 20). Right and left-of the origin of each arm, within the body

Distribution in time of Asteridea (fig. 17).-The Asteridea are disk, on the ventral

represented in the Lower Silurian series of strata by the genera Edriaster, Palaaster (ranging to Carboniferous), Stenaster, Taniaster, and Urasterella; in the Upper Silurian by Glyptaster, Palasterina, Paleocoma, Petraster, Palmipes, Lepi daster, and Trochitaster; in the Devonian by Aspidosoma, Ptilonasler, Asterias (also in Carboniferous), and Helianthaster; in the Carboniferous by Schanaster and Cribellites; by Pleuraster in the Trias; by Tropidaster in the Lias; and by Astropecten with other still living genera in the Lias and Oolites. The Cretaceous strata are more especially characterized by species of the recent genera Oreaster, Astrogonium, Goniodis cus, and Stellaster.

Order III.-OPHIURIDEA. FIG. 17.-Fossil Asteridea. The 1 Lepidaster Grayl, Forbes; Brittle-stars (fig. 18) have a general 2. Proton, Salter; U. Silurian, Dudley. external resemblance to the Aste- L. Ludlow rock, Salop. ridea. The body consists of a central disk with five or more simple or less usually ramifying rays, which are sharply distinguished from the disk, are without ambulacral grooves, and contain no prolongations of the stomach. plates, also hooks (considered to be the representatives of Spines and the pedicellarise of the Asteridea), are developed in the

adoral surface; B, abegroove; e, facet for tentacle.

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side, is an elongated ossicle, which in the Euryalida unites at the margin of the disk with an arched piece running towards the centre of the dorsal surface. The mouth is in the centre of the ventral face, and at each of its angles is a pair of tentacles. It leads into a simple sac-like alimentary Fra. 20.-Mouth skeleton of Ophioderma canal, which is without longicauda. (After Müller.) canal, the walls of groove for nerve-ring; m, peristomial plates; anus. The madreporic & first vertebral ossicles; e, interambulacral which are strengthened o, pale angulares.

piece of oral angle; f, torus angularis; A

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interradially situated scuta buccalia on the ventral side of with calcifications, leads from the surface of one of the the disk into a circular ambulacral canal, upon which rest minute plates, the homologues of the Holothuridean 'caland corresponding in position to the madreporic canal, there careous ring. Opening into the circular ambulacral canal,

are usually four interradially placed Polian vesicles. The necks of the Polian vesicles and the ambulacral ring give off the diverticula termed by Simrock vasa ambulacralia cavi. From the ambulacral ring proceed, the five radial canals between the superambulacral plates and the axial ossicles. In front of each ossicle they give off right and left branches to the pedicels. These are tentacle-like, devoid of basal vesicles, and, except in the Euryalidæ, pass out through openings between the superambulacral and lateral plates. The nervous system consists of an oral ring, which supplies a branch to each arm, running superficial to its radial ambulacral canal. Between the nerve and the latter is a neural canal. The genital organs are pairs of racemose glands attached to the inner dorsal surface of the disk; their products are shed into the perivisceral cavity, whence they make their way through the genital clefts between the origins of the arms.

Hermaphrodism has been observed in the species Ophiolepis squamata, and in some genera, as Ophiocoma and Ophiactis, scissiparous reproduction occurs. According to Lütken, this at an early age is more especially exhibited by the six-rayed forms. In certain cases development takes place within the egg, without any free pseudembryonic stage; but most generally metamorphosis from a plateiform larva takes place. The bilaterally symmetrical skeleton of the pluteus consists of eight radially diverging calcareous rods. The development of the embryo commences with the production of two cylindrical solid bodies, one on each side of the gullet, which form cellular blastemic masses, one behind and another in front of the stomach, and a third to the left of the pseudostome. The ambulacral system of the adult is developed from the last of these, which unites with the mass in front of the stomach to form the ventral portion of the body, whilst the posterior mass furnishes the dorsal portion.

The Ophiuridea may be classified as follows:

Sub-order I.-OPHIURIDE. Arms unbranched; ambulacral furrows covered with plates; genital clefts ordinarily five; habit creeping.

A. Oral clefts armed.

(i.) No papilla angulares. OPHIODERMATIDE.

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Buccal scutes trigonal disk granulated. Ex. Ophiura (Ophioderma), Ophiochata, Ophiopsammus. OPHIOLEPIDE. Buccal scutes pentagonal; disk scales naked. Ex. Ophiolepis, Ophioceramis, Ophiopus. AMPHIURIDE. Disk rugged and scaly; ray-plates spinous. Ex. Amphiura, Ophiacantha, Ophiopolis, Ophiostigma, Ophiactis. OPHIOMYXIDE. Disk naked; rays clothed with soft integument. Ex. Ophiomyxa, Ophioscolex. (ii.) Papilla angulares present. OPHIOCOMIDE. Dísk covered with solid plates. Ex. Ophiocoma, Ophiomastix, Ophiarthrum.

B. Oral clefts unarmed.

OPHIOTHRICIDE. Radial plates very large. Ex.
Ophiothrix, Ophiocnemis, Ophiogymna.

Sub-order II.-EURYALIDE. Arms simple or ramified, and capable of being rolled up towards the mouth; ambulacral furrows covered by soft integument. Spines are not present, but there are tufts of papille on the ventral surface of the arms; genital clefts ten.

ASTROPHYTIDE.

Astrophyton, Trichaster, Asteronyx, Asteromorpha, Asteroporpa. Distribution in time of Ophiuridea. The following Paleozoic genera are commonly referred to the Ophiuridea:-Protaster (Lower Silurian); Palaodiscus, Acroura, and Eucladia (Upper Silurian); Eugaster (Devonian of New York). Ophiura (1) occurs in the

Carboniferous Limestone of Russia. In the Muschelkalk occur

Aspidura and Aplocoma, and in higher Secondary strata Ophioderma, Ophiocoma, Amphiura, and other genera.

The Ophiuridea and Asteridea possess, in a marked degree, the power exhibited by the whole of the Echinodermata of reproducing lost portions of the body. The former have received the name of "Brittle-stars" on account of the remarkable facility with which species of the genus Ophiocoma not merely cast away their arms entire, but, at will, rapidly break them into little pieces. The same property has been noticed in the genus Luidia among the star-fishes, in the Crinoidean genus Comatula, and the Synaptide

among the Holotburidea. Writing of a species of Luidia, Prof. E. Forbes remarks: "The first time I ever took one of these creatures I succeeded in getting it into the boat entire. Never having seen one before, and quite unconscious of its suicidal powers, I spread it out on a rowing bench, the better to admire its form and colours. On attempting to remove it for preservation, to my horror and disappointment, I found only an assemblage of rejected members. My conservative endeavours were all neutralized by its destructive exertions, and it is now badly represented in my cabinet by a diskless arm and an armless disk." Major Fred. H. Lang relates (Nature, Oct. 12, 1876), that during a dredging expedition in Torbay, presuming on the fact that as a rule he could take up the specimens of Comatula rosacea and Ophiocoma rosula he had captured without occasioning their dismemberment, he "put about a hundred of the two sorts them;" for on reaching home he found "that both into a sponge-bag; but this was asking too much of Feather-stars and Brittle-stars had converted themselves into a mass of mince-meat! It would have been difficult to find a single portion of an arm a quarter of an inch long."

Order IV.-CHINOIDEA.-The body in the Crinoidea is cup-shaped or bursiform, and its base always in the young state and usually in the adult is attached by the apical pole either directly or, as more by commonly,

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of the inferior wall. Fig. 21.-Pentacrinus caput-Medusa. (After At the border of the calyx are 2-18,

Guttard.)

usually 5, arms or brachia, which are movable, and can be closed together over the oral disk (fig. 22). Between them, commonly in the centre of the disk, is the mouth, and near it, in one of the interradial spaces, is the anus.

It has been shown by Sir Wyville Thomson, (Phil. Trans. vol. clv. pt. 2), that the skeleton of Antedon rosaceus may be divided into two systems of plates, the radial and the perisomatic, the former including the articuli of the stem, the centro-dorsal plate, the radial plate, and the joints of the arms and pinnules, and the latter the basal, oral, and anal plates, and the interradial and other plates or spicule developed in the disk-membrane. "The body of

the pentacrinoid is at first, while yet included within the pseudembyro, and during its earliest fixed stage, surrounded and inclosed by plates of the perisomatic system alone." The predominance of the perisomatic system in the calyx of the older Crinoids and forms allied to them is hence a fact of considerable interest to the embryologist.

Goldfuss.)

The stem is made up of numerous ossicles articulated and interpenetrated by elastic fibres and soft connective tissue. It is attached at its distal end by a root-like expansion, or by numerous, filamentous, branched cirri, having joints similar to those of the stem. Other and unbranched cirri are attached in whorls to many of the ossicles of the stem. Through the centre of the stem runs a canal containing a soft solid substance. In the adult Antedon, as has been pointed out by Dr Carpenter (Proc. Roy. Soc., 1876), the medullary portion of the Crinoidal axis passes up through a FIG. 22.-Cyathocrinus pentangular five-chambered dilatation tuberculatus. (After of its cortical portion within the centrodorsal plate, and, reaching the cavity of the calyx, forms the pedicle by which chiefly the basal or dorsal surface of the visceral mass is attached to the calyx. The pedicle enters into the axial canal and passes through out its entire length, more or less imbedded in its walls, to the commencement of the subtentacular canals, where it apparently becomes continuous with the generative plexus of the disk. The stem varies in length, being short in Apiocrinus, long in Pentacrinus; it is round and sometimes moniliform in most Paleozoic Crinoids, but in Platycrinus compressed as it nears the calyx; in Woodocrinus it tapers from the calyx downwards. In the Paleozoic Crinoids the articulations of the ossicles radiate from the central canal, which is larger than in the more modern forms. In the Mesozoic genera the articular facets are commonly united by crescentic or stellate ridges. In the Silurian genus Periechocrinus the ossicles of the stem are alternately thicker and thinner. The calyx, which may be regarded as formed of the uppermost ossicles of the stem, is composed of several series of plates. The lowest of these is commonly formed by 2-6 pieces, the basalia, which in Rhizocrinus appear to be represented by a single central plate. The basalia in the Tesselata are succeeded by the parabasalia or sub-radialia; after the basalia or these come one or more rows of plates (in Rhizocrinus three), the radialia (see fig. 23). In Pentacrinus the radialia seem to form the commencement of the calyx. Supported by the radialia are series of arm plates, or brachialia, from the uppermost of which, as in Pentacrinus, may bifurcate the palmaria. The ossicles of the arms are sometimes single, sometimes united by syzygies, or immovable sutures. In the Silurian genera Anthocrinus and Crotalocrinus the subdivisions of the arms are very numerous, and by their lateral articulation form web-like expansions.

d

segments of the pinnuies borne by the second; and the calycine cavity is completed by the perisome uniting the Where, as in the Articulata, basal segments of the arms. represented by the modern Crinoids, the disk is more or less membranous, four or five deep furrows radiate upon its surface from the mouth, which pass on to the oral surface of the arms and extend to the extremity of their pinnules. They carry the hollow ambulacral tentacles, which pass out through pores in the perisome. In 1865 Mr J. Rofe demonstrated (Geol. Mag., ii. 245) in the case of seve ral genera of Tesselate Crinoids from the Mountain Limestone (Actinocrinus, Amphoracrinus, Cyathocrinus, and Platycrinus) that the groove on the upper surface of the arms divides at their base into two channels-(1) a superior channel passing up beneath (in some cases partly within) the plates of the dome or disk to its apex, and there uniting with an internal circular aperture, probably the mouth; and (2) an inferior channel which goes direct into the visceral cavity. These channels, since their discovery by Mr Rofe, have been shown to be generally present in the Tesselate Crinoids. The superior channels, on the supposition that the central opening is a mouth, doubtless served for the supply of food and of water for respiration; whilst the inferior channels probably gave passage to the motor muscles of the arms, and placed the visceral cavity in connection with the ovaries, if the latter, as in modern Crinoids, were situated in the arms. In the Paleozoic genus Rhodocrinus the arm is cylindrical, and without a groove on the upper surface, but immediately below its base is situated the orifice of a passage which turns upwards under the dome. What in the Paleozoic Crinoids is commonly regarded as the anal opening, is situated at the extremity of a proboscidiform tube (fig. 24) interradially placed, and often of great length-as much as 4 inches in Poteriocrinus plicatus. In existing Crinoids there are two apertures in the disk-the mouth, usually central, as in Rhizocrinus, and the interradially situated The mouth is closed by lobes of the perisome, the oral valves, which may contain calcareous plates. Between these run the oral or ambulacral grooves from the mouth to the arms. In Antedon (Comatula) the alimentary canal passes obliquely downwards from the mouth, then horizontally, and after more than a complete turn bends upwards again, and ends in a rectal chamber terminating in a spout-like prominence. Between the exterior of the mucous wall of the alimentary canal and its peritoneal covering is the intramural space. The double wall of the canal is strengthened by calcareous disks; and it is by the folding of the inner side of the wall, and the resultant piling

[graphic]

anus.

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together of layers of these plates that FIG. 24.-Dendroenthe vertical columella is produced. The nus longidactylus. body cavity is lined by a smooth A, calyx; B, proboscis, peritoneal membrane. The ambulacral furrows are bordered

The arms of Poteriocrinus plicatus FIG. 23.-Dissection of by plates, the ambulacral or marginal lamella, as in

calyx of Lecanocrinus macropetalus (after

Hall).

c, subradialia succeeding the

bifurcate 4 times, giving 80 rays; the total number of plates in that species has been estimated at 1300 (see J. G. Grenfell, Rep. Brit. Assoc., 1875, central basalia; d d p. 65). In the calyx of the Tesse- radialia; f, interradiala. lata there are plates, interradialia, present between the radialia. In Antedon the central portion of the viscera is contained in a basin formed by the 1st, 2d, and 3d radialia, and by the 1st and 2d pairs of brachialia, and the basal

Rhizocrinus and Pentacrinus, or, as in Antedon, by elevated ridges of the perisome, produced at the edge into a series of small lobes or valvules, and having grouped on their inner side the pedicels. The epithelial floor of the grooves, there is good reason to believe, is lined with cilia, which, like those of the gullet, serve to create currents in the water and thus to bring into the mouth Diatomaceæ, spores of Algae, minute Entomostraca, and other nutritive material. In Antedon, as has been shown by Dr Carpenter

25. -Section of

arm of Antedon rosa

ceus. (After Carpenter.)

(fig. 25), the tentacles communicate at their bases with a common trunk, the tentacular canal. Beneath this, but having no communication with it, lies the subtentacular canal, which is usually divided by a more or less incomplete septum. Each of the subtentacular canals is continuous with a branch of the axial canal, which communicates with the deeper portion of the perivisceral cavity by means of a minute pore m. situated nearly at the centre of the lower surface of the visceral mass, and partly occupied by the pedicel before referred to. As the axial canal extends downward through the visceral mass it comes into contiguity with the Fra. alimentary canal, and opens into it by irregular passages. There is in the arms a third canal, the coeliac, which is a continuation of the body-cavity or cœlom, and is separated from the subtentacular canal by a transverse partition. At the junction of this partition with the septum of the subtentacular canal there is a passage, the ital canal, in which lies the cellular cord known as the generative rachis, in connection with the visceral genital tissue. Enlargements of the rachis in the pinnules constitute the genital glands of the Crinoidea, the products of which may or may not be discharged by special orifices. Towards the extremity of the pinnules, in Antedon, the partition between the cœliac and subtentacular canals thins away, and becomes finally obso lete, thus possibly affording a means for the circulation of the nutritive fluid of the body, the subtentacular canals constituting an arterial or distributive and the cœliac a venous or collective system of vessels.

off lateral branches to te, tentacular canal, giving

the saccular (sensory?) organs, s, 3; stc, the two subtentacular canals; gr, genital rachis; cc, coeliac

canal; m, muscles; a,

organic basis of calca

reous segment; b, solid cord from quinquefocular organ.

From the arms the tentacular canals proceed inwards to unite with a circular canal situated around the gullet, and having connected with it numerous short processes similar to the vasa ambulacralia cavi of the Ophiuridea (see page 635). There is no madreporic tubercle, and the madreporic canal is apparently unrepresented in the Crinoidea. Respiration seems to be effected by the tentacles, and in Comatula also by the access of water through pores in the oral perisome, communicating with a series of sinuses 'below its under surface by means of funnel-shaped canals. The central organ of the nervous system in Antedon, according to Carpenter, is the dilated cortical portion of the axis of the stem within the centro-dorsal plate, which supplies branches to the cirri and the arms, and corresponds probably with the axial sheath which, in Pentacrinus, sends off cords at the nodes of the stem into the whorls of cirri (see fig. 21). A fibrillar band underlying the epithelial Hoor of the brachial furrows is regarded by Ludwig as a nerve "an afferent rather than a motor nerve" (Carpenter).

The development commences with the formation from the egg of an oval morula, which acquires four hoop-like ciliated bands, and a posterior terminal tuft of "cilia. An endodermal sac or archenteron results from an invagination of the blastoderm between the third and fourth ciliated bands; and from this three diverticula, two lateral and one ventral, take their rise, the remainder of the archenteron becoming an alimentary cavity communicating with an anterior cesophagus. The lateral diverticula are transformed into peritoneal sacs, one on the dorsal the other on the ventral side of the alimen

tary cavity, and their walls coming in contact produce a circular mesentery. In the ventral diverticulum the ambulacral vessels have their origin. Around the alimentary cavity, when the pseud-embryo is scarcely a line in length, there are formed two circles, each of five calcareous plates, which eventually become the oral and basal ossicles of the calyx. From the centre of the posterior circle extends a row of eight calcares rings, the future stem of the Crinoid, surrounding a backward prolongation of the dorsal

peritoneal sac. At the posterior extremity of the row is a cribriform disk, by which the young Crinoid subsequently attaches itself. The sarcodic body of the pseud-embryo begins to shrink, the pseudostome

and the two lower bands of cilia disappear, and afterwards the two upper bands, and the embryo then becomes fixed to a stone, seaweed, or some other object. A new mouth is formed in the centre of the disk by the separation of the oral plates, and the intestine by the production of a diverticulum of the alimentary cavity. In the early Pentacri noid stage of Comatula the basals rest upon the centrodorsal segment, but become at length metamorphosed into a single piece, the rosette; and the centro-dorsal segment by degrees increasing in size, the first radials come to rest upon its enfolded lip. During the same period, after the formation of an anus, the oral and basal plates disappear. The development of the dorsal cirri takes place as the proximal joint of the column enlarges to form the centro-dorsal piece. At the end of five or six months, when about an Comatula, natural size and magniinch in diameter, the young Comatula detaches itself from its stalk, and is then able to swim by means of its arms. The Pentacrinoid larval form of Comatula (fig. 26), previous to his discovery of the ultimate stages of its growth, had been termed by Vaughan Thompson Pentacrinus europaus. The Crinoidea are classified as follows:

FIG. 26.-Pentacrinoid larval forms of

[graphic]

fied.

Order I. TESSELATA. Calyx completely formed of calcareous plates, oral face without ambulacral furrows.

Family. Tesselata. Ex. Cyathocrinus, Actinocrinus. Order II. ARTICULATA. Oral face of calyx usually membranous or sub-membranous, with ambulacral furrows.

Family 1. Pentacrinida. Always attached. Ex. Pentacrinus, Rhizocrinus.

Family 2. Comatulida. Attached only in the young state. Ex. Antedon, Phanogenia. The Cretaceous genus Marsupites appears

to have been unattached.

The Crinoidea are represented by Glyptocrinus, Eucalyptocrinus, Marsupiocrinus, Taxocrinus, Ichthyocrinus, Periechocrinus, Cupres socrinus, Poteriocrinus, Woodocrinus, Cyathocrinus, Rhodocrinus,

5

FIG. 27.-Fossil Crinoidea.

1. Crotalocrinus rugosus, Mill.; U. Silurian, Dudley.

8

2. Poteriocrinus (joint of column); Carboniferous, Yorkshire.

3. Encrinus entrocha: L. Muschelkalk, Germany.

4. Apiocrinus Parkinsoni, Mill.; Bradford Clay.

5. Pentacrinus basaltiformis, Mill.; Lias, Lyme.

6. Marsupites ornatus, Mill.; Chalk, Sussex.

7. Comatula Glenotremites (upper surface of body),

8. Comatula (lower surface); Chalk, Sussex.

9. Eugeniacrinus quinquedactylus, Schl.; Oxfordian, Würtemberg.
10. Bourgueticrinus ellipticus, Mill.; Chalk, Kent.

and numerous other genera in Paleozoic strata, where their remains, especially in the Carboniferous series, are often the chief constituents of vast masses of compact limestone.' From their form the insulated articuli of the stem have come to be known as entrochi, screw

stones, or wheel-stones, and in the north of England, as "St Cuth

1 On Crinoidal Limestone, see J. Rofe, F.G.S., Geol. Mag., . p. 262.

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