ECHINODERMATA. 622 ECHINODERMATA. wood, and the free-swimming crinoids alniost fall anions the pelagic animals or plankton. Classification. The Echiiiodermata are di- vided among the following seven classes: Holo- thuroidea, Echinoidea, Asteroidea, Ophiuroidea, Crinoidea, Cystoidea, and Blastoidea. The last three groups, comprising mostly attached forms, are often ineliidcd as subclasses under a class Pcliiiatozoa. Wo prefer to associate the first four in a sub-brauch Eleutherozoa, and the last tlivce in a second sub-branch, Peliuatozoa. The characters of the various classes are as follows: I. HoLOTlU'KoiDEA ( liolothurians, or .sea-cu- cumbers). The body is cylindrical or worm-like, and has a soft, leathery skin, in which are im- bedded minute calcareous spicules or plates. A certain degree of bilateral symmetry is apparent, but there is little trace of radial pentameral symmetry. The mouth is at the anterior end of the body and is surrounded by tentacles; the anus is at the posterior end. The only portions of holothurians that are apt to be preserved as fossils are the calcareous bodies of the skin. These are known in rocks of Carboniferous to i-ecent time, but they are never common. See HOLOTHIRIAN. II. EciiixoiDE.v (sea-urchins). In this group the body is inclosed in a more or less rigid box of polygonal stony plates, arranged in rows that form five ambulacral and live interambulacral areas. In the more regular, radial forms both the mouth and the anus are central ou opposite surfaces of the body. In other irregular forms, exhibiting less radial and greater bilateral sym- metry, the mouth may be placed toward the an- terior end and the anus is variously situated, either on the dorsal, lateral, or ventral surface, but always in the middle line. The majority of' echinoids have complicated jaws. About 300 living and 12500 fossil species are known. The earliest appear in Ordovician rocks. See Echi- noidea; Sea-I'kcuix. III. AsTEROinEA (starfish). The body of the animals of this class are flattened dorso-ven- trally, and the ambulacra are produced into five (or some multi|)le of live) radiating arms. The arms are not at all demarcated from the central portion of the body, and they contain prolonga- tions of the ccelome and of most of the organs. Radial symmetrj- is quite marked. The tough skin contains calcareous plates of irregular form that are not united to each other, so that the body-wall is to some extent flexible. The ambu- lacral plates occupy the larger portion of the under sides of the arms, and the tube feet are well developed. The large mouth-opening is in the centre of the ventral surface of the disk, and the small anal opening is near the centre of the dorsal surface, except in a few genera, where it is wanting, and llie intestine is then closed. The earliest starfish appear in rocks of Ordovician age, and they have continued, though never be- coming abundant, through all subsequent periods down to the present time. See Starktsii. IV. Opiiiteoidea (brittlestars). The animals of this group have the body flattened in the same manner as in the Asteroidea. with the ambu- lacral arms produced into five long rays that are either simple or many times branched. The body is covered by stony plates that present a greater degree of systematic arrangement than is seen in the Asteroidea. and they are more firmly united. The central disk is well demar- cated from the arms, and no portions of the cadomic cavil_v or diverticula of the alimentary canal or genital system are prolonged into them. The arms serve for locomotion. The mouth is central on the ventral surface and the anus is closed. The class makes its first appearance in Silurian rocks, and it is represented, though rarely, in rocks of all later periods. At present its members are widely distributed. See OlMIIVKOlDEA; BaSKETFISII; BrITTLESTAR. The sub-branch Pelmatozoa. including the Crinoidea, Cystoidea, and Blastoidea, consti- tutes a group of mostly attached echiiiodcrms in which the body is inclosed in a somewhat cup- shaped, spherical, or sac-like bo.x of calcareous plates, and is raised, during at least a portion of the life of the individual, u])on a stalk with the ventral surface and mouth upward. V. CiiixoruEA ( sea-lilies >. These are pelma- tozoans in which the cup-shai)ed body-wall is m;ide up of closely joined plates arranged accord- ing to definite plans of pentameral radial sym- metry, the arrangemeut varying in every family according to the number of plates in the 'cal-x' or body-cup. The calyx bears five simple or branched, highly flexible arms, that are provided with pinnules to screen food from the water. The mouth is at t!ie middle of the upper or ventral surface, and the anus is excentric. Both these orifices undergo peculiar and important modifications that are used in classifying the members of the group. The class appeared in Ordovician time, and was exceedingly aln'.ndant during the Paleozoic era. Abimt '2000 fossil species, in I'ii genera, and 600 living .species are known. See Ckixoidea. . VI. Cystoide.v. Pelmatozoans with spherical or sac-like body inclosed in a more or less ir- regular box. composed of more or less closely united polygonal plates that present definite plans of arrangement only in the most advanced forms. Tlie animals were sessile, stalked, or free. The arms are poorly developed, and in some forms are not separated from the oral surface of the body, in which case they appear as ambu- lacral grooves radiating from the mouth. The mouth is central, and the anus is always excen- tric and is placed either on the oral or aboral surface. In some forms the mouth is slightly excentric. The earlier and more primitive mem- bers of the class are spherical with the plates very irregular. There are some genera which exhibit tendencies toward the crinoid type, others lesemble the blastoids, or the echinoids or star- fish. On this account the cystoids have by some authors been considered to resemble, more nearly than any other class, if not actually to represent, the original primitive eeliinoderm stock from which was derived through evolution all the other classes. Cystoids appear in the rocks of I'pper Cambrian age. being the earliest group of cchinoderms, and they reached a considerable expansion in the Ordovician and Silurian periods. They disappeared at the close of the Paleozoic. See Ctstoidea. VII. Blastoidea (Pentremites). This is a small extinct, specialized group of eehinoderms, derived from the cystoids at perhaps an early Ordovician date, that did not reach its maxi- mum development until Carboniferous time. Then, however, after the cystoids had quite dis- appeared, and the crinoids had begun to decline, this group attainr.l <cinsiileral)le prominence, so
