Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/656

634 E C II I N O D E K M A T A to the interior by fine closed tubuli 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 Pterastcr militaris hatches its young in a special pouch on the dorsal surface. The larva on leaving its egg is oval, but subsequently assumes a penta gonal 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-iish leaves the maternal pouch. In other cases the breeding-chamber may be formed by the bringing together of the bases of the rays, and the ciliated embryo develops at its anterior end club-shaped tubercles, by which it can attach itself to the breeding-chamber or to submarine objects. Until these processes appear the breeding- chamber remains closed. In general, the larva of the Asteridea begins life as a lobed and ciliated pseud-embryo, a common form of which is the Bipinnaria. Another form, the Brachiolaria, is distinguished principally by three tuberculated processes at the anterior end of the body. The ambulacral vessels of the adult are developed in the pseud-embryo from a portion of one of the diverticula of the stomach in which originate the peritoneal cavity and the whole or great portion of the mesodermic structures. The Asteridea are classed by 11. Edrnond Perrier as follows : DIVISION I. Pedicellarire pedunculated ; pedicels (except in Labidiastcr and Pedicellastcr} quadriserial. ASXERIIIXE. Ex. Asterias (Astcracanfhion), Ueliaster, Calvasterias, Anasterias, Labidiastcr, Pedicellastcr. DIVISION II. Pedicellarise sessile; pedicels ordinarily biserial. i. Dorsal skeleton reticulate. ECHINASTEKID^B. Ex. AcantJiaster, Solaster, Ecliinastcr, Cribrella. ii. Dorsal skeleton of longitudinal series of rounded or quad rangular ossicles ; integument generally granulated. LINUKIAD.E. Ex. Ophidiaster, LincJcia, Scytaster. iii. Skeleton, at least of lower surface, of tesselated ossicles ; dorsal and ventral marginal plates very distinct. GONIASTERID^E. Ex. Pcntagonastcr, Goniodisciis, Goni- aster, Culcita, Asterodiscus, Choriastcr. iv. Skeletal ossicles imbricated; with spines on the free border, or rounded and completely covered with small spines. ASTERINIIXE. Ex. Palmipes, Asterina, Nepanthia. V. Skeleton of paxillse. ASTROPECTINID.E. Ex. Chcctaster, Luidia, Astropcctcn, Archastcr, Ctcnodiscus. vi. Dermal investment supported by spines radiating from the prominent skeletal ossicles. PTERASTERID,E. Ex. Pteraster. vii. Arms long, straight, distinct from disk, with minute spines on dorsal surface. BiiisixGiD.fi. Ex. Brisinga. Distribution in time of Asteridea (fig. 17). The Asteridea are represented in the Lower Silurian series of strata by the genera Edriaster, Palce- aster (ranging to Carboniferous), Stcnaster, Tceniastcr, and Urasterclla; in the Upper Silurian by Glyptaster, Palccasterina, Palceocoma, Petraster, Palmipcs, Lcpi- daster, and T rod Master ; in the Devonian by Aspidosoma, Ptilonccstcr, Asterias (also in Carboniferous), and Hclianth- tister ; in the Carboniferous by Schcen~ aster and Cribellitcs ; by Pleurastcr in the Trias ; by Tropidaster in the Lias ; and by Astropccten with other still living genera in the Lias and Oolites. The Cretaceous strata are more especially characterized by species of the recent genera Oreaster, Astrogonium, Goniodis- cus, and Stellaster. Order ///. OPHIURIDEA. FIG. 17. Fossil Asteridea. The 1. LepidHster Grayi, Forbes; i>nttle-StarS (fioj. 18) havp T rrpnpral U - Silurian, Dudley. pvf-prnol r Ti general 2 rrotaster Mntoni Salter . external resemblance to the Aste- L.Lu<iiowroc*,Suiop. ridea The bo Jy consists of a central disk with five or more ess usuaUy ramifying rays, which are sharply dis tinguished from the disk, are without ambulacral grooves ani contain no prolongations of the stomach. Spines and plates, also hooks (considered to be the representatives of the pedicellarue 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 FIG. 18. Ophiopholis bellis, upper surface. dorsal median or antambulacral row, and two side rows of spinous imbricated plates. More rarely the perisome of the arms is leathery in consistence, and bears small plates, 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 ri^ht 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, cor responding 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 lG r A 7^i ??** le J i e , , , . . , oi Ophiolems. (After couple ol mterambulacral pieces, with M tiller.) the inner edges of which is articulated A, adorai surface ; B, abc- a single ossicle, the torus angularis. g^oovef* facet" fo^ten- The last-mentioned bears the papilla. tacle - angulares, and beneath these the pake angulares, which are short flat processes, moved by muscles, and serving as teeth (see fig. 20). Bight and left of the origin of each arm, within the body disk, on the ventral side, is an elongated ossicle, which in the Euryalidce 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 FlG - 20. Mouth skeleton of Ophioderma canal, which is without anus. The madreporic rf - f e ^^ 1 ^ b Mgtef C / iora8 n an^Sari8* cr ^ Canal, the Walls of proove for nerve-ring; m, peristomial plates; which are strengthened palic angul1 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 cal careous ring. Opening into the circular ambulacral canal,

and corresponding in position "-o the madreporic canal, there