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.—...