29S 



ECHINODERMA 



forms. The cardiac division gives origin to five gastric pouches which 

 can be protruded from the mouth or retracted by appropriate muscles. 

 The gonads are five pairs of racemose glands lying in the basis of the arms 

 and opening interradially between the arms. Lastly, in the ccelom is the 

 stone canal (accompanied by the lymphoid glands, and with it enclosed 

 in the axial sinus) extending from the aboral madreporite to the ring 

 canal near the mouth. 



The radial nerve, canal and blood-vessel, which start from the cir- 

 cumoral rings, lie in the roof of the ambulacral groove between the 



ambulacra. The nerve, lying in the ecto- 

 derm, ends at the tip of the arm in a 

 compound eye spot colored with red or 

 orange pigment which experiment shows 

 is sensitive to hght. A second nerve has 

 been described lying in the coelom of the 

 arm. The ambulacral system corre- 

 sponds with the foregoing description (p. 

 292), the ampulte as well as the five or 

 more Polian and Tiedemann's (racemose) 

 vesicles projecting into the coelom. 



Since the arms contain nearly all im- 

 portant organs, their physiological independ- 

 ence is easily understood. Arms broken off 

 not only live, but regenerate first the disc and 

 then new arms which appear at first like small 

 buds (comet form, fig. 285). This separation 

 of arms may occur through accident, or it not 

 infrequently is produced by the animal itself. 

 AsTERiD.5:, well developed arms, four 

 rows of ambulacra; Asterias,'^ Lcptaslerias* 

 H chaster* (numerous arms). Sol.-\sterid/E, two rows of ambulacra, arms 

 sometimes numerous; Pythonasicr (fig. 290). Asteeinid.e, arms short or 

 body is pentagonal, no large plates on the margins of the arms. Astcriscus 

 (fig. 288). In other forms {Cidcita* fig. 286, Hippasicria* Ctcnodiscus'^) 

 the body is more or less pentangular, margin with large plates. 



Fig. 2QO. — Pythonaster murrayi 

 (after Sladen). Oral view show- 

 ing ambulacral grooves. 



Class II. Ophiuroidea (Brittle Stars). 



In these, as in the Asteroidea, there are disc and arms, the latter some- 

 times branched, but the internal anatomy is different. The ambulacral 

 plates have been drawn inside the arm and each pair fused to a large 

 'vertebra' (fig. 291). As a result the ccelom of the arms is greatly re- 

 duced, the hepatic ca^ca are lacking, and the alimentary canal, which 

 lacks an anus, is confined to the disc. By the ingrowth of ventral plates 



