Zöology; a textbook for colleges and universities . nd some of them are largely due to lime-secret-ing algae or seaweeds. CHAPTER TWENTY-NINE Origin andcharactersof Echino-dermata ECHINODERMATA I. THE origin of the Echinodermata is problematical,but they are certainly much less primitive than theCcelenterata. The larva is more or less wormlike orcuriously branched, with a distinct bilateral seems to be a certain relationship with the Cirri-pedia or barnacles, and therefore with the this may be, the phylum is one of the most dis-tinct and easily recognized, tho
Zöology; a textbook for colleges and universities . nd some of them are largely due to lime-secret-ing algae or seaweeds. CHAPTER TWENTY-NINE Origin andcharactersof Echino-dermata ECHINODERMATA I. THE origin of the Echinodermata is problematical,but they are certainly much less primitive than theCcelenterata. The larva is more or less wormlike orcuriously branched, with a distinct bilateral seems to be a certain relationship with the Cirri-pedia or barnacles, and therefore with the this may be, the phylum is one of the most dis-tinct and easily recognized, though its different membersare very diverse. They inhabit the sea, although one ofthe wormlike sea cucumbers (Synapta) may be found inbrackish water in mangrove swamps. The adult ani-mals are usually recognizable by their radial symmetry,with a calcareous outer skeleton; internally we find acomplete alimentary canal, with two openings, and abody cavity between this and the outer wall. The nerv-ous system is closely connected with the skin, and there. Draining by W. P. Hay FIG. 50. Common starfish (Aslerias) of Atlantic Coast. A, upper or aboral sur-face ; B, lower or oral surface; C, cross-section of one of the arms; D, diagram ofthe water-vascular system; m, madreporic body; e, eye; mo, mouth; ag, ambula-cral groove ; /, tube feet; rt, radial water tube; /, digestive gland; b, body cavity;j, plates of skeleton. 218 ECHINODERMA TA 219 is no brain. There is no heart or definite system ofblood vessels. There is, however, a remarkable water- Water- r , , vascular vascular system, which consists ot a series 01 tubes con- system nected with tube feet or podia, especially conspicuous inthe starfish, where they serve for locomotion. In astarfish or sea urchin a sievelike plate (madreporite) maybe found on the upper (aboral) surface. Through thiswater passes into a canal, propelled by movements ofminute cilia. This canal or tube ends in a tubular ring,from which proceed radially five tu
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