Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/651

ECHINODERMATA (from ex<Vos, a hedgehog or sea- urchin, and oepfJM, skin), a class of marine animals which constitutes with the class Scolecida the sub-kingdom Annuloida of Huxley, or, according to some authorities, is a distinct sub-kingdom of the Invertebrata. Familiar ex amples of the Echinodermata are the Sea-urchins, Star fishes, 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 perlsome or dermis develops a calcareous skeleton of numer ous 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 con sisting 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, echinopcedium, or, as it has been termed by Sir Wyville Thomson, the pseud-embryo, pro duced from the egy 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 pro cesses of the same. In the A&teridea 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 pro cesses. 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 ambula cral 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 ambu lacral 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 Turbel- laria and Tceniada 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.) Edrioasterida; (VII.) Blastoidea; (VIII.) Holothuridea, or Sea-slugs. Of these orders V., VI., and VII. have been extinct ^since the Palseozoic period. By some authorities the Edrioasterida are included with the Cystidea.

Fig. 1.—Psammechinus esculentus.

Fig. 2.—Eotula augusti.}}

Fig. 3.—Echinus gracilis.}} a, ainbulacral plates ; 6, poriferous zone ; c, inter- ambulacral plates. (After Agassiz.)

Fig. 4.—Stcnrerhinus intermedius.

a, portion of ambulacral area; 6, poriferous zones; e. two interambulacral plates; d, primary tubercles. (After Wright.)

Order I.—Echinidea.—The body in the Echinidea is

spheroidal, oval, discoid, or heart-shaped, and the shell, test, or pensome bears numerous spines. A common European type of the group is the species Psammechinus (Echinus, L.) esculentm (see fig. 1). In certain forms (ScutelMae] the test is perforated by slit-like apertures, and curiously lobed or digitate (fig. 2). With few exceptions the test is a rmd structure of numerous plates united by their edges. In the 1 Echinothurida 3 , however (Galveria,Phormosomo., and the extinct Eckinotkuria and Lepidocentrus], and also in the Palse- ozoic genera Archceocidaris, Lepidestlies, and Lepidech inusr, the plates of the corona overlap, so as to resemble the peristo- mial plates of Cidaris. The plates are composed of a dense calcareous network, consisting chief ly of calcium carbonate. As the test is invested with an epidermis, and is produced mainly by calcification of the meso- derm, it is to be re garded 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 interambu lacra. In the Palaeozoic forms, which con stitute the sub order Perischoe- chinidoeof. M 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