ECHINODERMATA (from eχιvoς, a hedgehog or sea-urchin, and δip∕zα, skin), a class of marine animals which constitutes with the class Scolecida the sub-kingdom Annuloida υf Huxley, or, according to some authorities, is a distinct sub-kingdom of the Invertebrata. Familiar examples of the Echinodermata are the Sea-urchins, Starfishes, Feather-stars, and Sea-cucumbers of the coasts of Britain. The characteristics of the group may be briefly summarized thus. The adult presents a more or less marked, although never perfect, radial symmetry of parts; the larva, in most instances, is bilaterally symmetrical. The perisome or dermis develops a calcareous skeleton of numerous interlocking plates or of detached plates or spicules. The muscular tissue consists chiefly of unstriped fibres. The intestinal canal terminates in a distinct anal aperture. An aquiferous or ambulacral system of organs, regarded as homologous with the water-vascular system of the Scolecida, is generally present; and there is a nervous system consisting of a ganglionated circular or polygonal cord, which surrounds the oesophagus, and sends off’ branches parallel with and superficial to the ambulacral canals. The sexes are in the majority of cases distinct, and the reproductive organs are generally placed symmetrically with respect to the radially disposed skeleton,
In all Echinodermata of which the life-history has been worked out, the larva, echinopaedûtm, or, as it has been termed by Sir Wyville Thomson, the pseud-embryo, produced from the egg is, with but one or two exceptions, ovoid, free-swimming, and provided with cilia, which become after a time confined to one or more bilaterally symmetrical bands running transversely or obliquely to the long axis of the body, and frequently borne on processes of the same. In the Asteridea and Holothuridea the larva is vermiform and devoid of skeleton; in the Echinidea, it is pluteiform (Latin, pluteus, a pent-house, or breast-work), and has a continuous calcareous skeleton, passing into and affording support to the body processes. A stomach, with an oesophagus and intestine, which make with each other an angle open towards the ventral side of the body, is early developed in the Echinoderm larva. The peritoneal cavity and ambulacral system of vessels are developed from diverticula of the alimentary canal. A tube formed by an involution of the integument of the pseud-embryo to one side of the dorsal line may remain connected with the ambulacral system of the adult as the madreporic canal. In the Echinidea, Asteridea, Ophiuridea, and Crinoidea the body-wall of the adult is formed from the blastema; the larval body, more or less of the intestine, and, when present, the skeleton are cast off or absorbed into the new organism, and another mouth appears in the centre of the circular vessel. It is by this peculiar metagenetic mode of development of the Echinoderm within its larva that the class Echinodermata is specially allied to the orders Turbellaria and Toeniada of the class Scolecida.
The Echinodermata may be divided into the following orders :—(I.) Echinidea, or Sea-urchins; (II.) Asteridea, or Star-fishes; (III.) Ophiuridea, or Sand-stars; (IV.) Crinoidea, or Feather-stars; (V.) Cystidea; (VI.) Edrio-asterιda; (VII.) Blastoidea; (VIII.) Holothuridea, or Seaslugs. Of these orders V., VI., and VII. have been extinct since the Palaeozoic period. By some authorities the Edrioasterida are included with the Cystidea.
Order I.— Echinidea. —The body in the Echlnidea is spheroidal, oval, discoid, or heart-shaped, and the shell, test, or perisome bears numerous spines. A common European type of the group is the species Psamrnechinus (Echinus, L.) eκw⅛ ι w (see fig. 1). In certain forms (Scutellidoe) the test is perforated by slit-like apertures, and curiously lobed or digitate (fig. 2). With few exceptions the test is a rio r id structure of numerous plates united by their edges. In the Echinothuridce, however (fjαlveria, Phormosoma, and the extinct Echinothτιria and Lepidocentrus), and also in the Palaeozoic genera Archoeocidaris, Lepidestlιes, and Lepidech iηus 9 the plates of the corona overlap, so as to resemble the peristomial plates of Cidaris. The plates are composed of a dense calcareous network, consisting chiefly of calcium carbonate. As the test is invested with an epidermis, and is produced mainly by calcification of the mesoderm, it is to be regarded as an internal shell or endo-skeleton.
In the typical recent echini the walls of the corona or main body of the shell, when freed from spines, are seen to consist of five zones or areas, the ambulacra (Latin, ambulacrum, a walk), composed of double rows of pentagonal plates, and alternating with five other double rows, the iηterambu lacra.
In the Palaeozoic forms, which constitute the suborder Perischoe-chinidοsοl M i Coy, the interambulacrum is made up of more than two rows of plates, of which the intermediate and central are hexagonal in form (see figs. 3 and 4). In the genera Melonites and Oligoporus there are extra ambulacra! as well as interambulacral plates. The ambulacra, which are usually narrower than the interambulacra, have near their outer edge small shield-like spaces, «m&ones, bounded by a more or less elevated wall, and perforated by pairs of small orifices or pores for the protrusion of the feet or pedicels, each pair of pores corresponding to one of the pore plates—the primitive ossicles which commonly unite to form the ambulacral plates. The ambulacra are either homogeneous, i.e., composed of similar elements gradually diminishing in size towards the poles of the test, or (as in the Spatangoida and most of the Clypeastroida) are heterogeneous, having the upper portion petaloid in shape, and the lower with pores scattered in areas not always confined to the ambulacral plates, or arranged in ramifying fasciae. In the Spatangoida the anterior unpaired ambulacrum is commonly obsolete (see fig. 5). In the Oolitic genus Dysaster, the two postero-lateral ambulacra, forming the bivium, are separate from the rest, and converge over the anal opening; while the three anterior, the trivium, unite at the apical disk (see fig. 8). The growth of the urchin in length is effected by the formation of new plates at the apical end of the corona, and in breadth by additions to the margins of the plates. On the surface of the plates are tubercles of different sizes, each with a knob or elevation, sometimes crenulated, by which the acetabulum of the spine is attached (see figs. 4 and 5). The presence or absence in the tubercle of a central perforation for the passage of a ligament for the spine is an important distinguishing character in various groups of fossil echini. The spines in the young state are ciliated; like the plates of the test they are composed of a calcareous network, and are interpenetrated and covered by the perisome, which contains the muscular fibres by which they are moved. They are short in the Clypeastroida and Spatangoida, and of various lengths in the Hchinoida, and offer a considerable diversity of form and ornamentation. Dr Gray {Ann. of Nat. Hist., i. p. 414) mentions the discovery in Sicily of the fragment of a spine of an echinus, the circumference of which was nearly 1∣ inches, and tho length more than 8 inches. In Porocidaηs purpurata, a deep-sea form, the spines are paddle-shaped, and very flat, and are serrate on the edges; in Coelopleurus the long curved spines resemble the antennae of certain beetles. Scattered over the surface of the test, and more especially on the oral membrane, are the pedicellariae, generally regarded as peculiarly modified spines; these, when well developed (fig. G), consist of a long flexible stem, furnished at the summit with a forceps of three pincers or prongs, which snap together, and seize firm hold of any object that comes in their way. They serve for the removal from the neighbourhood of the shell of dirt of all kinds, and apparently also for defence. Calvema fenestrata has pedicellariae with four valves. In some Spatangoid genera the corona bears symmetrical bands of minute tubercles with attached spines, the stmitoe or fas-cioles, distinguished, according to their position with respect to the anus or to the apical or the marginal teι minations of the petaloid ambulacra, as sub-anal, circum-anal, intrapetalous, and peripetalous. The spines of the semitae have a thick integumentary covering, and except at the enlarged apex, are closely studded w ith cilia. Lovén has shown, the occurrence, in all Echinidea but Cidaris, of another kind of appendages of the test, possibly sensory organs, to which he has given the name of sphoeridia. These are button-like, spheroidal bodies, seldom above ι⅞Ι>^h inch in length, furnished with a short stalk, and normally articulated with small projecting tubercles on the plates of the ambulacra and peristome. Sometimes they become concealed by a layer of the test, in which there remains only a fine external fissure. At the summit or apical pole of the test is a space occupied by the ocular and genital plates, which in the Hchinoida (Endocyclica) encircle the anus with its anal plates. The fhe genital plates, which are opposite the interambulacra, or interradial in position, are perforated by apertures for the exit of the reproductive products. In the Clypeastroida and Spatangoida (Exocyclica), in which the anus is eccentric, and exterior to the apical disk, one of the genital plates is usually imperforate (fig. 7). The five ocular plates are situated radially, crowning the apical ends of the ambulacra; on the surface of each is a depression, having a pore for an ocellus or eye-spot. Always, except in the Clypeastridoe, the right antero lateral genital plate, or, in other words, that situated to the right of the anterior ambulacrum of the trivium, is larger than the others, and bears the convex, perforated madreporic tubercle or madreporite. In the Clypeastroida this is most frequently extended over the other apical plates. The hinder genital plate, with apparently one exception, is wanting in the Spatangidoe, its place being occupied by the madreporite. The Palaeozic Echinidéa differ from the more modern forms by the greater number of perforations of their ocular and genital plates. At the base of the test is the mouth with its buccal membrane and plates. The species Leskia mirabilis (the type of the sub-family Leskiadoe, family Spatangidoe) has both mouth and anus closed by converging triangular valves. In the Hchinoida and Clypeastroida the mouth is central in position, and provided with teeth; in the Spatangoida it is eccentric and edentulous. The teeth resemble those of Rodents in form, and are arranged in hard wedge-shaped sockets or alveoli, which by their union form a pentagonal cone. As the outer substance of the tooth is harder than the inner, it is less readily worn away, and thus always presents a sharp edge. Each alveolus is composed of two halves united in the middle line, and each half, again, consists of a superior and inferior portion. The alveoli are inter-radial in position, or opposite the interambulacra. They are connected by transverse muscular fibres, and alternate with superiorly placed, thickish, radial structures, the rotuloe or falces, which, in the Hchinoida, bear each a bifurcated piece, the radius. This skeletal mouth-apparatus is commonly known as “Aristotle’s lantern.” The calcified internal arched processes termed auriculae, at the oral end of the ambulacra in the Echinoida (of the interambulacra in Cidaris), regarded as homologous with the internal ambulacral ossicles of the Asteridea and Ophiuridea, are formed each of two pieces. From the top of the auriculae pass retractor muscles to the outer edge of the alveoli. The oral skeleton is provided also with protractors proceeding from the alveoli to the lower interambulacral edge of the corona, besides special muscles connected with the radii. In the Clypeastroida arched (in Cidaris unarched) processes are given off from the ambulacral plates, at the sides of the ambulacral canals; and in the Clypeastroid genus Scutella the dorsal and ventral walls of the corona are connected by vertical calcareous plates or trabeculae. None of the above-mentioned internal calcareous processes is developed in the Spatangoida. The mouth communicates by a tortuous oesophagus with the stomach. The intestine forms a sort of festoon on the inner side of the shell, and is attached to it by a mesentery.
Of the internal organs of the Echinidea the most important and characteristic are those constituting the ambulacral system. These are (1) a circular or slightly pentagonal vessel placed around and traversed by the oesophagus, on the inside of the alveoli, and between the nervous and blood-vascular rings; (2) caecal appendages of this vessel, called Polian vesicles (absent in the Clypeastroida and Spatangoida), answering to the racemose appendages of the Holothuridea; (3) the membranous or calcareous madreporic canal, termed also the sand-canal, which runs nearly vertically through the axis of the body, and, communicating with the exterior by the madreporic tubercle, supplies water to 1; (4) vessels radiating from 1 along the parietes of the body, and opening eventually in t O basal sacs, or ampullae, proceeding from the canals of the pedicels shortly above their origin. In the vessels of the ambulacral system is contained a watery fluid strained from the perivisceral cavity. The pedicels, which may vary considerably in shape, are tubular structures, usually terminated by a sucking-disk; they have contractile, muscular walls, and are capable of being protruded beyond the extremities of the spines. They subserve locomotor, tactile, or branchial functions. The corpusculated perivisceral fluid is kept in motion by the cilia clothing the lining membrane of the body and the viscera, Where modified pedicels or ambulacral gills are absent, as in the Echinoida, the Cidαridαe excepted, aeration of that fluid is apparently promoted by branchial developments from the peristome, the hollow stems of which communicate with the body-cavity. The nervous system consists of a slender, pentagonal, red or violet hoop around the gullet, superficial to the circular ambulacral vessel, with five ganglia sending off as many cords, which, passing out between the alveoli, take a course similar to that of the ambulacral radial canals, giving off fine side branches which pass in their course through the ambulacral pores, probably supplying the pedicels, spines, aud pedicellariae, and terminate eventually in the pigmented eye-spots. The principal vessels of the pseud-haemal or blood-vascular system of the Echinidea appear to be two trunks, the one on the dorsal, the other on the ventral side of the alimentary canal; these, according to Hoffmann, communicate either directly or by a distinct trunk with the water-vascular ring.
From an examination of Echinus sphoera, Psammechinus miliaris, Tοxopneustes lividus, ' and Amphidetus cordatus, Perrier determined that, as maintained by Hoffmann, the circulatory and aquiferous systems are identical; that the so-called “heart” is only a gland, which opens by a canal.into a funnel-shaped space bounded by the lining membrane of the test and the madreporite; that the artery proceeding from the water-vascular ring is distributed upon the first loop of the intestine, forming there ramifications which unite with those of the intestinal vein; and that the vein has no communication with the water-vascular ring, but is connected by ten branches and by its two extremities with a collateral canal, which floats freely in the perivisceral cavity beneath the intestine. Further, he found that the ambulacral vessels and their branches terminate blindly, the circulation consisting simply in a to-and-fro movement.of their contents.
The reproductive organs are large racemose glands, situated beneath the upper termination of the interambulacra, and opening externally by the genital pores. The sexes are distinct. The spermatozoa have vibratile filaments; the egg is fecundated after leaving the body of the female, and in about eight hours undergoes complete yelk-division.
The pseud-embryo or echinopaedium, at first ciliated and spheroidal, becomes after a time wedge-shaped; at its broad end appears the mouth or pscudostome, and at the other the anus or pseudoproct. Simultaneously with these the skeletal rods and ciliated bands of the pluteus begin to be produced. The development of the ambulacral system commences with the formation of a sac which lies to the left of the junction of the pseud-embryonic gullet and stomach, and is prolonged into a canal opening by a pore on the dorsal surface of the larva. The blind end of the sac becomes a quinque-petaloid rosette, from which radiate the ambulacral vessels; a new mouth is formed in the centre of this, at the bottom of a depression in the integument of the pseud-embryo, and the canal of the sac becomes the madreporic tube. The skeleton of the pluteus separates, as development proceeds, into several pieces, and is by degrees discarded, whilst its processes atrophy, and the body assumes the rounded form of the embryo urchin. This, however, has in many cases to undergo sundry important changes before its resemblance to the adult is complete. Thus in the young of species of Spalangus the peristome is almost central, and is pentagonal in form. In the Echinoida, before the appearance of the anus in the embryo, its place is occupied by a single plate, the sub-anal, and the anus appears near its border, towards the posterior right ambulacrum. It lies within a circle formed by five imperforate pieces, the future genital plates, and these again aie surrounded by five imperforate ocular plates, with intervening ambulacral plates. The central anal plate persists in the family Salenidoe among the Echinoida. If it be considered as homologous with the dorso-central plate of Marsupitcs, or the basalia of the calyx in other Criuoids, the genital plates correspond to the ρarabasalia, and the ocular to the first radialia.
The food of the Echinidea consists either of seaweed, and small shell-fish and crustaceans, which are conveyed to the mouth by the pedicels, or, as in the case of the edentulous forms,, of sand and earth containing nutritive material. In the species Anochanus sinensis, one of the Cassidulidcc, Grube discovered the presence of an incubatory chamber at the apical pole of the test, containing embryos in various stages of their growth. Certain species, as observed by Cailliaud, Deshayes, and Lory, have the power at a very early age of drilling for themselves burrows in the hardest rocks, such as granite and grit.
Allusion is made to the echinus in the writings of Aristophanes, Horace, Martial, and other classical authors. By the ancients it was considered a delicacy, and the common species Psammechinus (Echinus, L.) esculentus, Ag., especially in spring, when the ova are matured, is still eaten in some parts of Europe. Sir Thomas Browne, in his Vulgar Errors, mentions a notion formerly current that the spines of the echinus were a remedy for the stone, and “films in horses’ eyes.” They are put to practical use in some countries as slate-pencils.
Various systems of classification have been adopted for the Echinidea. In the following scheme the principal groups are arranged chiefly according to the position of the anal opening and the nature of the ambulacra.
Sub-order I.— Tesselata. Echinidea with interambulacra of more than two rows of plates.
Sub-order II.— Typica. Echinidea with interambulacra of but two rows of plates.
Group L— Regularia or Endocyclica. Mouth central or subcentral; anus usually central and opposite the mouth, and never exterior to apical disk.
A. Anus central.
i. Shell round.
Cidaridoe. Tubercles smooth, perforate, peristome unnotched; anal plates ten; auriculae unclosed; buccal branchiae absent.
Diadematidoe. Tubercles crenulate, perforate, peristome notched, spines hollow. Allied forms are the fossil Hcmicidaridοe.
Arbaciadoe. Tubercles smooth, imperforate; four large anal plates; auriculae closed.
Bchinidae. Tubercles imperforate, or perforate and crenulate; anal plates numerous; pairs of pores in ranks of three, four, or more.
ii. Shell oval or elliptical.
Echinometridoe. Pores in ranks of five or six pairs.
B. Anus eccentric through intervention of one or several supernumerary apical plates.
Salenidoe. All fossil forms, with exception of Salenia rarispina.
Group II.— Irregularia or Exocyclica. Anus eccentric, not within the apical disk.
A. Ambulacra simple, not petaloid.
Galeritidoe. Mouth central; shell globular or subpentagonal; a single apex at which the ambulacra converge.
Dysasteridoe. Mouth eccentric; shell ovoid or heartshaped; two apices, at which the bivium and trivium respectively converge.
B. Ambulacra more or less petaloid.
i. Dental apparatus present.
Clypeastridoe. Shell more or less flattened, sub-pentagonal.
Scutellidoe. Shell depressed, discoidal, often digitate or perforated; lower surface with ramifying grooves.
ii. Dental apparatus absent.
Cassidulidoe. Mouth central or nearly so; peristome sub-pentagonal.
Spatangidce. Mouth eccentric, transverse or reniform.
To the group Regularia must be added the Cretaceous and Recent family of Echinoihuridoe. The Echinidea are represented in Palaeo-
zoic strata by the Tesselata or Perischoechinidae, Paloeekinus (see 1 in ng. 8), PerιschodomUs t Lepideehinus, Eocidaris, Archoeocidaris
(2 in fig. 8), Melonites, Oligopοrus, and Lepidesthes (see Quart. Jour. Geol. Soc., XXX. 307). The Echinidee and Dysasteridoe occur first in the Trias, and are represented by numerous species in Mesozoic strata; the Salenidoe, Galeritidoe, and Cassidulidoe make their earliest appearance in rocks of Jurassic, and the Spatangidce, including the sub-family Anunehytidoe, in rocks of Cretaceous age. The accompanying figures represent fossil forms of Echinidea characteristic of various strata. Some account of the distribution in space of the Echinidea, together with that of other classes of the Echinodermata, will be found at page 278 of the present volume. See also Sir Wyville Thomson, Proc. Roy. Soc., χχ. 1872.
Order II.— Asteridea. —The Asteridea, or Star-fishes, have mostly a star-shaped body, composed of a central disk and five or more rays. The common British species of Solaster, S. papposus (fig. 10), has ordinarily 13 rays:
>S. helianthoides, a South American species, has as many as 34, the extinct >S. Moretoni of the Great Oolite had 33, and another fossil species, of Devonian age, Helianthaster Rhenanus, had 16 rays. The rays are sometimes very short (fig. 11), or altogether wanting, the body having the form of a pentagonal disk. In the Brisingidoe they may attain a length of many feet. The perisome in the Asteridea is coriaceous, and consists of an ectoderm with a thin ciliated cuticle, a muscular mesoderm which, contains calcareous skeletal plates or ossicula, and an 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 paxilloe. The pedicellariae 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.
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 middleof 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 lower or outer ends to rows of plates, the adambulacral ossicles, which form the margins of the grooves, and are themselves succeeded by one or more series of marginal ossicles (fig. 12). The outer ends of the innermost pairs of ambulacral ossicles unite round the mouth to form five crests, which bear spines and pedicellariae. 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 these there are intervertebral and interambulacral longitudinal muscles. The ambulacral grooves are nearly filled with the tube-feet 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 four-ranked (fig. 14). They are formed by notches or semipores incised one on the distal and the other on the oral surface of each ambulacral ossicle, and lying alternately external and internal to one another in position on successive ossicles (fig. 15). The mouth, which is devoid of dentary apparatus, is situated in the middle of a membranous disk in the centre of the oral surface.
It leads by a short gullet into the stomach. The 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 caecal 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 Lηiclia 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 (Oρhidiaster, Edwηαster) 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. Respiration appears to be effected by means of water supplied to the interior by fine closed tubu!i or dermal branchice between the plates of the perisome. In Solaster water can 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 Pter aster 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 breedingchamber 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 Bipiηηaria. Another form, the Brachiolaria, is distinguished principally by three tuberculated processes at the anterior end of the body. The ambulacra! 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 nιesoderιnic structures.
The Asteridea are classed by Μ. Edmond Perrier as follows :— Division I. Pedicellariae pedunculated; pedicels (except in Labidiaster and Pedicellaster} quadriserial.
Asj'eriidae. Ex. Asterias (Asteracanthion), IIeliaster, Calvastcrias, Anasterias, Labidiaster, Pedicellaster. Division II.— Pedicellariae sessile; pedicels ordinarily biserial.
i. Dorsal skeleton reticulate.
Echinasteeidae. Ex. Acanthaster, Solaster, Echinaster, Crιbrella. ii. Dorsal skeleton of longitudinal series of rounded or quad rangular ossicles; integument generally granulated.
Linckiadae. Ex. Ophidiaster, Linckia, Scytaster. in. Skeleton, at least of lower surface, of tesselated ossicles; dorsal and ventral marginal plates very distinct.
Goniasteridae. Ex. Pentagonaster, Goniodiscus, Goni∙ aster, Culcita, Asterodiscus, Choriaster. iv. Sk detal ossicles imbricated; with spines on the free border, or rounded and completely covered with small spines.
Asterinidae. Ex. Palmipes, Asterina, Nepanthia.
V. Skeleton of paxillae.
Astropectinidae. Ex. Choetaster, Luidia, Astropeeten, Archaster, Gtcnodiscus. vi. Dermal investment supported by spines radiating from the prominent skeletal ossicles.
Pterasteridae. Ex. Pteraster. vii. Arms long, straight, distinct from disk, with minute spines on dorsal surface.
Brisingidae. Ex. Brisinga.
Distribution in time of Asteridea (fig. 17).—The Asteridea are represented in the Lower Silurian series of strata by the genera Edriaster, Palοe∙ aster (ranging to Carboniferous), Stenaster, Taeniaster, and Urasterella; in the Upper Silurian by Glyρtaster, Palaeasterina, Palaeocoma, Petraster, Palmipes, Lepi-dastcr, and Trochitaster; in the Devonian by Aspidosoma, Ptilonaster, Asterias (also in Carboniferous), and Helianthaster; in the Carboniferous by Schaen-aster and Cribellitcs; by Pleuraster in the Trias; by Tropidaster in the Lias; and by Astropeeten 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, Goniodiscus, and Stellaster.
Order III.— Ophiuridea. The brittle-stars (fig. 18) have a general external resemblance to the Asteridea. The body consists of a central disk with five or more simple or less usuaTy ramifying rays, which are sharply distinguished from the disk, are without ambulacral grooves, and contain no prolongations of the stomach. Spines and plates, also hooks (considered to be the representatives of t ιe pedicellariae of the Asteridea), are developed in the perisome. The dermal skeleton of the arms is constituted usually of a ventral or superambulacral row of plates, a 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, of which the ventral are the largest, and perforated with a double line of pores. 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 vessel and a nerve. The innermost of 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 thejoαpi‰ angιιlares, and beneath these the f>aloe 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 disk, on the ventral side, is an elongated ossicle, which in the Euryalidoe 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 canal, which is without anus. The madreporic canal, the walls of which are strengthened with calcifications, leads from the surface of one of the interradially situated scuta buccalia on the ventral side of the disk into a circular ambulacral canal, upon which rest minute plates, the homologues of the Holothuridean calcareous ring. Opening into the circular ambulacral canal, and corresponding in position t o the madreporic canal, there 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 caνi. From the ambulacral ring proceed i 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 Euryalidae, pass out through openings between the superambulacral and l iteral 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.
Hermaphrodisni lias been observed in the species Ophiolepis squamata; and in some genera, as Ophioeoma and Ophiaetis, 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 pseud-embryonic stage; but most generally metamorphosis from a pluteiform larva takes place. The bilaterally symmetrical skeleton of the pluteus consist's 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.— OPHIURIDAe. Arms unbranched; ambulacral furrows covered with plates; genital clefts ordinarily five; habit creeping.
A. Oral clefts armed.
(i.) No papilloe angulares. Ophiodermatidae. Buccal scutes trigonal; disk granulated. Ex. Ophiura {Ophioderrna), Ophio- choeta, Ophioρsammus. Ophiolepidae. Buccal scutes pentagonal; disk scales naked. Ex. Ophioleqris, Ophioceramis, Ophiοpus.
Amphiuridae. Disk nigged and scaly; ray-plates spinous. Ex. Amphiura, Ophiacantha, Ophiopholis, Ophiostigma, Ophiaetis. Ophiomyxidae. Disk naked; rays clothed with soft integument. Ex. Ophiomyxa, Oρhioscolex.
(ii. ) Papilloe angulares present.
Ophiocomidae. Disk covered with solid plates. Ex. Oρhioeoma, Ophiornastix, Oρhiarthrum.
B. Oral clefts unarmed.
Ophiothriciiee. Radial plates very large. Ex.
Ophiothrix, Ophiocnemis, Ophiogymna.
Sub-order II.— EURYALIDAe. 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 papilloe on the ventral surface of the arms; genital clefts ten.
Astrophytidae. Astrophyton, Trichaster, Asteronyx, Astcromorpha, Asteroporpa.
Distribution in time of Ophiuridea.—The following Palaeozoic genera are commonly referred to the Ophiuridea :— Protaster (Lower Silurian); Palawdiseus, Acroura, and Eueladia (Upper Silurian); Eugaster (Devonian of New York). Ophiura (?) occurs in the Carboniferous Limestone of Russia. In the Muschelkalk occur A>pidura and Aplocorna, and in higher Secondary strata Ophio-d'.rrna, Ophioeoma, 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 Ορhiocoma 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, i ι the Crinoidean genus Comαfw. , σ, and the Synaptidoe among the Holothuridea. 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 Ophioeoma rοsula he had captured without occasioning their dismemberment, he “put about a hundred of the two sorts into a sponge-bag; but this was asking too much of them;” for on reaching home he found “that both 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.”
OτderIE.— Ckinoidea. —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 commo uly, by means of a calcareous stem to submarine objects. The inferior or dorsal wall of the body or calyx is formed of polygonal plates articulated by their edges, and the superior or ventral face or disk, which may be either flat or arched, is formed either by a perisomal membrane, occasionally strengthened with scattered calcifications, or, as in the Tesselata, by regularly arranged plates, tegmvnal·ia, resembling those of the inferior wall. At the border of the calyx are 2-18, 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 Wy ville Thomson, {Phil. Trans, vol. civ. 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 spiculae developed in the disk-membrane. “ 'Die body of the pentacrinoid is at first, while yet included within the pseud-embyro, 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.
The stern 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 pentangular five-chambered dilatation 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 Apiocrimιs, long in Pentacrinus; it is round and sometimes moniliform in most Palaeozoic Crinoids, but in Plαtycrvηus compressed as it nears the calyx; in Woodocrinus it tapers from the calyx downwards. In the Palaeozoic 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 Periechocrl·mns 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 Tesselala 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 Crota-lοcrinus the subdivisions of the arms are very numerous, and by their lateral articulation form web-like expansions.
The arms of Poteriocrinus plicatus 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, p. 65). In the calyx of the Tesselata 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 segments of the pinnules borne by the second; and the calyci ne cavity is completed by the perisome uniting the basal segments of the arms. Where, as in the ΛrticwZαta, 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 Sir J. Rofe demonstrated (Geol. Mag., ii. 245) in the case of several genera .of Tesselate Crinoids from the Mountain Limestone (Actinocrinus, Amphoracmnus, Cyathocrvnus, 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 Palaeozoic 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 Palaeozoic 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 Poterwcmnus plicatus. In existing Crinoids there are two apertures in the disk—the mouth, usually central, as in Rhizocrinus, and the interradially situated anus. 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 together of layers of these plates that the vertical columella is produced. The body cavity is lined by a smooth peritoneal membrane. The ambulacral furrows are bordered by plates, the ambulacral or marginal lamell<ν, as in 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 Diatomaceae, spores of Algae, minute Entomostraca, and other nutritive material.
In Antedon, as has been shown by Dr 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 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 alimentary canal, and opens into it by irregular passages. There is in the arm3 a third canal, the coeliac, which is a continuation of the body-cavity or coelom, 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 genital 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 coeliac and subtentacular canals thins away, and becomes finally obsolete, 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 coeliac a venous or collective system of vessels.
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 am 1 >ulacralia caνi 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 Pentacrintis, sends off cords at the nodes of the stem into the whorls of cirri (see fig. 21). A fibrillar band underlying the epithelial floor 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 oesophagus. The lateral diverticula are transformed into peritoneal sacs, one on the dorsal the other on the ventral side of the alimentary 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 calcareous 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 Pentacrinoid stage of Comαtulα the basais 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 ⅛ an inch 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 Ρeηtacriwus europaeus.
The Crinoidea are classified as follows :—
Order I. Tesselata. Calyx completely formed of calcareous plates, oral face without ambulacral furrows.
Family. Tesselata. Ex. Cyathoerinus, Actinocrinus.
Order II. Articulata. Oral face of calyx usually membranous or sub-membranous, with ambulacral furrows.
Family 1. Pentaerinidae. Always attached. Ex. Pentacrinus, Rhizocrinus.
Family 2. Comatulidoe. Attached only in the young state. Ex. Antedon, Phanogenia. The Cretaceous genus Marsupites appears to have been unattached.
The Crinoidea are represented by Glyptocrinus, Eucailyptoerinus, Marsupiocrinus, Taxocrinus, Iehthyoerinus, Perieehocrinus, Cupres-sοcτh>ιus, Poteriocrinus, IVoodoerinus, Cyathoerinus, PJιodocriwus, and numerous other genera in Palaeozoic 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, screwstones, or wheel-stones, and in the north of England, as “St Cuthbert’s beads.” Among the various forms by which the Crinoidea were represented during the Mesozoic period, towards the close of which they diminished materially in number, are Encrinus (Muschelkalk); Extracrinus, Pentacrinus (Lias); Comatulidae (Rhaetic); Ayiocrinus, Millericrinus, Eugeniacrinus, and such forms as Saccosoma and Pterocorna, related to Comatula (Jurassic); and BourgτιeiicriniLS, Marsupites, and (?) Comaiulidae (Chalk). The Tertiary genera are but few. They include Bourg ueticrinus and Gainοcrinus (London Clay), and Comatula (Coralline Crag.). Pentacrinus, Rhizocrinus (allied to Aρiocrinus Bourgueticrinus) and the related forms Bathycrinus and IIyocrinus, the sessile genus Holοpus, Aetinometra, and Comatula are living forms.
Order V.— Cystiüea. —The body in the Cystidea is in most cases rounded, and is covered with polygonal plates; it is attached by a stem, and may be provided with arms or pinnules developed from the upper or oral surface. The stem is short, usually similar to that of the Crinoidea in construction, but without cirri; the joints are rounded, and sometimes moniliform, and usually become broader but thinner towards the base of the body. In Ateleocystites (see H. Woodward, Geol. Mag. 1871, p. 71) the calyx is compressed laterally, and shows sculpturing similar to that of the plates of the pedunculated Cirmpedia. The plates of the calyx are pentagonal, hexagonal, or imperfectly triangular, and are closely united together; they vary in number, and in Splκeronites (1 in fig. 28) are very numerous. In Cryp-tοcrinus the calyx is composed of three rows of plates, which may be regarded as basalia, parabasalia, and radialia. According to Professor E. Forbes (Mem. of the Geolog. Survey of Great Britain, 1848, vol. ii. part 2), the following series of plates may be generally distinguished :—a basal series; suboνaman, centrolateral, and supra-ovarian series on a plane below, on the same plane with, and on a plane above the ovarian pyramid respectively; circa-oνarian plates or ossicles, encircling that structure; and oral plates, immediately surrounding the mouth, which vary considerably in number (fig. 29). The plates are frequently ornamented with grooves and tubercles. Arms and pinnules are not universal. In Comarocgstites punctatus, Billings, the arms are free; sometimes they are wanting, and the pinnules are attached to the upper portion of tho calyx. Commonly the arms resemble ambulacra, and are reflected towards the base of the calyx, and closely applied to its surface. Pores on the antambulacral surface may be absent (Cryptocrinus), irregularly scattered (Caryocrinus), in pairs (Sphoeronites), or, as in Pseudοcrimus (2 in fig. 28), Echinoencrinus, and other genera, slit-like, and arranged to form “pectinated rhombs,” or “hydrospires,” the two halves of each rhomb being on separate plates.
In Cαryocriηus ornatus there are thirty pectinated rhombs, consisting each of a number of parallel internal flat tubes communicating at both ends with pores opening internally. The rhombs in Pleurocystites are not tubular as in Caryoeriwus, but are made up of numerous parallel inward folds of an exceedingly thin part of the test. Again in Paloeocystites tenuiradiatus the whole surface bears rhombs, which, when uninjured, have a complete though very thin calcareous covering, and communicate by a small pore at their base with the body-cavity. (See Billings, Ann. and Mag. of Mat. Hist. 1870, p. 259-61.)
What is usually regarded as the mouth is situated in the centre of the ventral surface of the calyx, opposite the point of attachment of the stem, and from it radiate the furrows for the arms, when those appendages are present. It is apparently small and circular in Sphoeronites ; in C'aryocystitβs it is transversely elongated and lobed; in IIemicosmites elevated on a proboscis; in Echinoencrinus usually longitudinal and bordered by peculiar plates. A small perforation alongside the mouth, considered to be the anus, is generally present. It has sometimes, as in Echinoencrinus (fig. 30) and Λpiocystites, the form of “an arched or crescentic groove terminating apparently at each end with a pore, and having united with it, or placed a little below it, an orifice in the middle line of a suture, as if in the junction of two oral plates” (Forbes). Almost invariably, on the oral portion of the body, interradially placed, is a round or oval aperture covered by a pyramid of 5 or 6 triangular valves. This in the opinion of most authorities is probably the ovarial orifice; but according to Mr Billings it is an oro-anal aperture, the central opening or mouth above mentioned being an “ambulacral orifice.” Cystidaι first occur in strata of Cambrian age. They especially characterize the Lower and Upper Silurian series of rocks, and apparently died out in the Carboniferous epoch.
Order VI.— Edrioasterida. —Under this head are grouped such forms as Edrioaster, Agelacrinites, and Hemicystites. The shape is that of a rounded star-fish or flattened echinus with a concave base. There is an ovarian pyramid, but stem and arms are wanting, and tho ambulacra communicate by perforations with the calycir.o cavity. The Edrioasterida are exclusively Palaeozoic. Their nearest living ally is the Australian species Hyponome Sarsii, Lov., which approaches Agelacrinites in form.
Order VII.— Blastoidea. —In this group of fossil Echinoderms the bud-shaped or prismatic, armless, and closely plated calyx is supported on a short, jointed stem. Of the three basal plates in Pentremites two are double; succeeding the basais is a row of five pieces, and into the deep clefts of the upper portion of these fit the lower ends or apices of the ambulacra; a third series of five small, deltoid, interradial plates occupy the spaces between the oral portions of the ambulacra. The ambulacra, or “pseudambulacral areas,” present a superior surface formed by a double series of ossicles running from a median line to the border, where they support pinnules; beneath the ossicles is usually a lanceolate plate formed in many if not all species of Pentremites of two contiguous plates, and edged by a simple row of transverse pieces, which are pierced with marginal pores.
Each row of pores opens below into one or more flat canals, or, according to the definition of Billings (Ann. and Mag. of Nat. Hist., vol. V. 4th ser. p. 263), into a “hydrospire” consisting of “an elongated internal sac, one side of which is attached to the inside of the shell [or test], while the side opposite, or towards the central axis of the visceral cavity, is more or less deeply folded longitudinally” (see fig. 32). These internal canals, as suggested by Rofe, may possibly represent the tubes under the dome of the Crinoidea. In Codonaster the ambulacra are confined to the upper portion only of the calyx. Pores and attached tubes are wanting; but there are striated structures between the arms, similar in appearance and probably also in function to the pectinated rhombs of the Cystidea, their ridges, as first shown by Rofe (Ceol. Mag., 1865, ii. p. 251), being the tops of a series of folds of a thin test or membrane, which were perhaps “respiratory sacs, lined with cilia, and constructed of a porous test, through which air from the water could pass by diffusion.” The expanded ends of the neighbouring tubes of each two ambulacra form at the summit of the test four double and two single apertures commonly termed “ovarian orifices;” between the two latter there is usually a third, apparently anal, opening. In Eleτdherοcmnus there are three paired, and two single pores only. The Blastoidea, which are (1) represented by P entremîtes in Upper Silurian strata, attained their principal development during the Carboniferous epoch, at the close of which they seem to have become extinct.
Order VHP— Holothurtdea. —The Holothuridea, Sea-slug3, Trepangs, or Sea-cucumbers (figs. 33 and 34) have a long, cylindroid, sometimes flattened body, which is without shell, and is brown to purplish-red in colour. The perisome, which is unciliated, is composed of three layers,— a structureless epidermis, a cellular dermis, and an internal elastic layer. The two interior layers contain calcareous spiculae, which vary much according to the species, and may take the shape of perforated disks, wheels, anchors, and hooks (fig. 35). Rarely the dorsal integument may develop an armature of overlapping plates (Psolus), which may bear spines (Echinocucumis). Underlying the perisome is a layer of circular muscular fibres, some of which pass into the mesenteries; a second internal set of five simple orpaired bands of longitudinal muscular fibres are attached at one end to the radial pieces of the calcareous oral ring, and supply branches to the oral tentacles, and at the other are inserted into the sphincter of the anus. It is by means of the longitudinal muscles that the Holothurid, when irritated, effects the discharge of its viscera at the hinder extremity of its body. In the midst of a circle of tentacles, five or multiples of five in number, is the mouth, which is without dental apparatus. The tentacles vary considerably in shape, and may be cylindrical, shield-shaped, pinnate, or ramified. They serve as organs of touch, of nutrition, and occasionally also of locomotion. The alimentary canal is simple, and usually longer than the body, so as to be two or three times folded on itself; it is attached to the interior surface of the body by mesenteries, and may terminate in a cloaca. Its w alls are composed of an external layer of circular, and an internal layer of longitudinal muscular fibres, and an innermost cellular lining. In common with the peritoneal surface of the body, it is ciliated. Two, or more rarely four or five, branched processes of the cloaca, the respiratory trees or water-lungs, are ordinarily present, and are connected to the body-wall by a mesentery or by threads (fig. 36). They appear to be excretory in function, water being continually passed into and out of them through the contractile cloaca. Their ultimate ramifications terminate in minute openings, by means of which they appear to have communication with the peritoneal cavity. In Echinοcucumis the respiratory trees are only singlebranched. In Synαpta, in which they do not occur, there are funnel-shaped ciliated bodies attached to the mesentery of the aliment-ary canal. The respiratory tree occupying the ventral left interradial space has been observed in many cases to be surrounded by a plexus of the pseudhaemal system of vessels. What are termed Cuvierian organs are appendages of the cloaca, which, according to Semper (Reisen im Archipel der Philippinen, i. pp. 139, 140), are muscular, and can be used as a means of defence, being capable of protrusion externally. The main trunks of the pseudhaemal system, which is often exceedingly complex, are two vessels, one on the dorsal, the other on the ventral face of the intestine, which are connected with each other-by capillary reticulations. The calcareous ring surrounding the gullet, already referred to, consists usually of five interradial, and five notched or perforated radial plates united by muscles—the homologues of the auriculae of the Echinidea. A circlet of plates, in certain species, surrounds the anus. The fluid filling the various canals of the ambulacral system contains nucleated cells. The ambulacral circular vessel lies behind the calcareous ring; it gives off from one to five or more Polian vesicles, also one or more madreporic canals. Calcareous spiculae are contained in the wall of the madreporic canal, and often it is terminated at the free end by a cribriform plate similar to a madreporite. By means of the madreporic canal the ambulacral vessels communicate with the peritoneal cavity. From the ambulacral ring there proceed five, sometimes more tentacular canals, which supply caecal branches to the tentacles, and usually, also, five ambulacral canals, which give rise to as many rows of pedicels situated in most cases radially, and forming a dorsal bivium and ventral trivium. Sometimes, as in Psolus, the pedicels are irregularly dispersed over the whole perisome, or they may be met with only on the lower surface of the body, where they subserve locomotion. In Molpadia and its allies there are ambulacral canals, but no pedicels; in the Synaptidoe the canals are wanting. The sexual organs are one or two groups of branched tubuli, which open either on the dorsal surface or between the two dorsal tentacles. Except in the Synaptidoe, and apparently also the Liodermatidoe, the sexes are distinct. The nervous system consists of a circular cord, lying above the ambulacral ring, and giving off five apparently hollow branches, which pass through holes or notches in the radial plates of the calcareous ring, to proceed down the centre of the longitudinal muscular bands of the body.
Development is direct in Holοthuriα tremula and Peηtacta doliolum; and a species of Synapta is viviparous. The majority of the Holothuridea undergo metamorphoses, during which, however, no portion of the echinopaedium is discarded. By invagination of the morula an intestinal cavity is produced, into which an ectodermic invagination opens, forming the upper portion of the alimentary canal. The cilia that at first clothe the body commonly become limited to a doubly bent band, the larva developing into an auricularia, which has sac-like processes, and occasionally spicules, but never a skeleton. Growing less transparent, the auricularia loses its lateral processes, the mouth disappears, and the larva reaches the “pupa-stage,” in which the body is surrounded by five ciliated hoops. A new mouth with tentacles is now developed, the ciliated bands are lost, and the animal attains the same shape as the adult. The peritoneal cavity and the ambulacral system both originate in a caecal process of the archenteron or primitive endodermal sac, which, separating from the latter, forms what has been termed the vaso-peritoneal vesicle. This gives off a process which opens on the dorsal surface, and which eventually furnishes the madreporic canal. A portion of the vesicle remains in connection with this process, and is transformed into the ambulacral vessels; the other portion becomes two sausage-shaped vesicles at the sides of the alimentary canal, above and below which they grow together, and form a continuous space, the future peritoneal cavity. The inner and outer wall of the chamber thus resulting become attached respectively to the parietes of the body and to the alimentary canal, and mainly contribute to the production of their peritoneal and muscular layers.
The Holothuridea are creeping in habit; some, however, of the Synaptidoe are able to swim. The apodal forms move themselves by contractions of the body, and by means of their tentacles. The Holothuridea derive their nourishment from the sand which they swallow, and from Diatomaceae, Foraminifera, and other minute marine organisms. Among the internal parasites of the Holothuridea are small fishes of the genus Fierasfer, embryos of the gasteropod Euioeoncha, mirabilis, and Copepod crustaceans (Pinnotheres). Certain species of Holothuria are much esteemed in China as food, and constitute an important article of commerce with that country.
The Holothuridea may be grouped as follows :—
I. APNΕUMONA. Bespiratory trees and Cuvierian organs absent; mouth and anus at opposite ends of the body; ambulacral canals five; hermaphrodite.
(1.) Synaptidoe. Pedicels absent. Ex. Synapta, Chirodοta.
(2.) Oncinolabidw. Pedicels present. Ex. Echinosorna.
II. TETRAPNEUMONA. Respiratory trees four; body flasklike; mouth and anus at the same end of it, the former surrounded by ten tentacles and ten calcareous plates, the latter by as many papillae and plates; five ambulacra diverge from the anal, and five from the oral region of the body; pedicels in two rows. Ex. Rlιοpalodina lageniformis only = the class Diplostomidea of Semper.
III. DIPNEUMONA. Respiratory trees two; Cuvierian organs present; mouth and anus polar; pedicels single-ranked.
(1.) Liodermatidoe. Pedicels absent; tentacles shield-like, cylindrical, or branched. Ex. Biosoma, Haplodactyla, Molgjadia.
(2.) Dendrochirotoe. Tentacles branched. Ex. Thycne, Phyllo-phorus, Ochnus, Psolus.
(3.) Aspidochirotae. Tentacles shield - like. Ex. Aspidochir, Stichopus, Sporadipus, Holothuria.
Mr C. Moore, F.G.S. (Rep. Brit. Assoc., 1872, ρ. 117), lias described wheel-like spiculae of four species of Chirodota, one from the Inferior Oolite, one from the Upper Lias, and two from the Middle Lias. “They are formed of a number of minute wheel-spokes, varying from 5 to 13, which start from a central axis, and are surrounded on the outside by a wheel-tire; on the inner edge of some species are a series of very minute teeth, extending over the central cavity.” Mr R. Etheridge, jun. (in the Memoirs of the Geol. Survey of Scotland, Explanation of Sheet 23, 1873), has called'attention to the discovery by Mr J. Bennie, survey-collector, of similar organisms in the Lower Carboniferous Limestone group of E. Kilbride, and in shales of the Upper Limestone group of Williamwood, near Glasgow.
Bibliographt. —The following are some of the more important treatises on the Echinodermata :—Tiedemann, Anatοrnie der Rährenholothurie, <tc.. Heidelb. 1820; Miller, A Natural History of Crinoida, 1821; J. V. Thompson, “On Pentacrinus europceusi' Edinb New Phil.Jσurn., xx., 1836; L. Agassiz, Monog. d'Echinodemes, <fcc., Neuchâtel, 1838-42,—see also Cornρt. rend., xxiii., 1846, xxv., 1847; L. Agassiz and Desor, “Catalogue Raisonné,” &c., Ann. sc. nat., 3d series, vi., vii., viii∙, 1846-47; E. Forbes, A Hi story of British Starfishes, &c., 1841; see also Mem. of the Geolog. Survey of Great Britain, ii., 1848; D'Orbigny, Hist, des Crinoïdes, 1841; J. Miiller, “lieber den Bau von Pentacrinus caput-Meduste," Abhandl. der Berl. Alad., 1841; Id., “lieber die Larven und die Entwickelung derEchino-dermen,” ib., 1846, 1848-52, 1854 (see Huxley in Ann. of Nat. Hist., 1851); Id., “Ueber die Gattung Comatula,” ib., 1847; Id„ Ueber Synapta digitata, <⅛c., Beilin, 1852; J.Muller and Troschel. System der Asteriden, Brunsw. 1842; De Quatrefages, “Mém. sur. la Synapte de Duvernoy,” Ann. sc. nat. 1842; Austin, A Monog. on Recent and Fossil Crinoida, 1844; von Buch, “Ueber Cystideen," Abhandl. der Berl. Ahad., 1844; Sars, “Mém. sur le développement des Astéries," Ann. sc. nat., 3d ser., ii., 1844; Id., ofversigt af Norges Echinodermer, Christiania, 1861; Id., Mém. pour servir à la connaissance des Crinoïdes vivants, Christ. 1868; Id., Remarkable Forms of Life, 1875; Gaudry, “Mém. sur les pieces solides chez les Stellérides,” Ann. des sc. nat., 1851; Römer, “Tlonog. der . . . Blastoideen," Archiv f. Naturgesch., 1851; Gray, “A Description of Rhopalodina,” Ann. of Nat Hist., 1853; Desor, Synopsis des Echinides fossiles, Paris and Weisbaden, 1855-58; wright, Monog. of British fossil Echinodermata from the Oolite, 1855-60; British oolitic Echinodermata, 1862-66; Monog. of the Cretaceous Echinodermata, 1S64; Liitken, Oversigt over Grönland s Echinodermata, Copenh. 1857; Id.. “Bidrag til Kundskabom Echinodei me,” Vidensk. Meddelels. Kjöbenhavn, 1S63; Id.,* l 0mves-tindiens Pentaeriner,” <fcc.. Naturh. Forenings Meddelels. Kjobenh., 1864; Id., * ’ ophiuridarum . . . descriptiones nonnullae,’’ Översigtover d. K.D. V. Selsk. For-hand., 1872; De Köninck and Le Hon, “Crinoïdes du terrain carbonifère de la Belgique,’’ Nouv. Mém. Acad. Belg., 1858; Cailliaud, “Sur les oursins perforants de Bretagne.” Revue et Mag.de . Zool., 1856, and Compt. rend., xlv, 1857,—see also Deshayes and Lory in Bull. Soc. Géol. , 1856; wi∣scn, “The Nervous System of the Asterida.” Trans. Linnean Soc., 1862; A. Agassiz, “Embryology of Echinoderms,” Mem. of the Americ. Acad., 1864, and Ann. sc. nat., 5th ser., i., 1865; Id., “Embryo-l∩gv of the Starfish," Contrib. to the Nat. Hist, of the United States, v., 1864; Id., Revision of the Echini, Cambr. U.S., 1872-73; A. Baur, Beiträge zur Naturgesch. der Synapta digitata. Dresd. 1864, Jena, 1865; Sir wyville Thomson, “Οη the Embryology of rtntedon rosaceus," Phil. Trans., 1865; Id., Depths of the Sea, 1872; W. B. Carpenter, “Οη the Structure . . . of Antedon rosaceus," Phil. Trans., 1866; see also Proc. Roy. Soc., 1876; Schulze, Monog. der Echinodermen der Eifeler Kalkes, Vienna, 1866'; Selenka, “Beiträge zur Anat. und Systematik der HolotJiurien,” Zeit sch. f.Wiss. Zoof., 1867-68; Semper, Reisen im Archipel der Philippinen, I.eipsic, 2 vols. 1868-76; Id., “Ueber Comatula,’’ Würzburg Arbeiten, Ï87 4: Mctschnikoff, “Studien ü. die Entwickelung der Echinodermen und Nemertinen,’’ Mém.Acad. St Pétersbourg, 1869; Perrier, “Sur les pédicellaires et les ambulacrcs des As-, téries et des Oursins,” Ann. sc. nat., 1869 and 1870; Id., “Sur ΓAnatomie de la Comatula rosacea," Arch, de Zoolog, expér., 1873; Id., “Sur l'Appareil circulatoire des Oursins,” ib., 1875; Id., Révision de la collection de Stellérides du Museum éTIIist. Nat. de Paris, with bibliography, 1875-76; Baudelot, “Contrib. à l’histoire du svstème nerveux des Écliinodermes,” in Bull, de la Soc. d Hist. Nat. de Strasbourg, 1870. and A rcΛ. de Zool. expér., ii., 1872; Greef, u Ucber den Bau der Echinodermen;” Marb. Sitzungsber., 1871-74; Hoffmann, “Zur Anat. der Echinen und Spatangen.” Niedert. Archiv f. Zool., 1871 and 1872; “Zur Anat. der Asteriden," zö., 1875; Laube, Die Echinoiden der oesterreichisch-ungarischen oberen Tertiärablagerungen, Vienna, 1871; Lovén, “On the Structure of the Echinoidea” (trans, by Dallas), Ann. and Mag. of Nat. Hist., 4th set·., x., 1872; Id., “Études sur les Échinoïdées,” Kongel. Sι∙enska Vetenskaps Acad. Handtingar, 1875; 0. Hertwig, “Beiträge zur Kenntniss der Bildung . . . der thierischen Eies,” Gegenbaur's morphol. Jahrb. 1875; P. H. Carpenter, “On the Anat. of £he Anns of the Crinoids,” Journ. of Anat. and Physiol., 1876: Teuscher, Beitiäge zur Anat. der Echinodermen,” Jenaische Zeitsch,, 1876; Simrock, “Anat. und Schizogonie der ophiactis virens," Zritsch. fi II jm. Zool , 1876. See also the papers and larger works of Allman, Billings, De Blainvιlle, Claus, Costa, Danιell-sen, Desmoulins,Düben, Dujardin. Frémonville, Gegenbaur,Gray, Grube, Haeckel, Henson. Huxley, King, Koren, Krohn, Liitken, Lyman, M l Coy, von Martens, Pictet, Pourtales, A. Schneider, Verrill, and others. (F. IL B.)
