THE ANIMAL KINGDOM 



439 



CLASS III. OPHIUROIDEA. Brittle stars. The arms are 

 sharply set off from the central disk and are without ambulacral 

 grooves. The ambulacra are tactile, respiratory and excretory 

 in function. The external surface is not ciliated. Hepatic 

 caeca are absent and the intestine lacks an anal opening. 

 Example: Ophiura robusta, a Cape Cod form (Fig. 259). 



AM 



Fig. 260. — A, Arbacia, a sea urchin, side view, am, ambulacra. B, oral view 

 showing mouth region surrounded by shorter ambulacra and blunter spines. 



CLASS IV. ECHINOIDEA. Sea urchins and sand dollars. 

 A subglobular or disk-shaped body without arms. Usually, 

 five calcareous teeth project from the mouth. The calcareous 

 exoskeleton is well developed and is often provided with long 

 movable spines. The surface of the body is ciliated. 





■•-.. 





, 



-> 



:;^* 



■ 







• • ■& %•.■■ 



.'§" ::':■ •..'■.■ ://■'% 



&- : 



■ '/. 





'V ■ ■ 



it ■.;• ■ .- ■■ ■*. ,:e;J^ 



v. 



Fig. 261. — A, Echinarachnius, a sand dollar, aboral view; B, oral view. 



Examples: Arbacia punctulata, a sea urchin common on the 

 Atlantic coast (Fig. 260); Echinarachnius parna, the sand 

 dollar (Fig. 261). 

 CLASS V. HOLOTHURIOIDEA. An elongated, wormlike 

 shape, with a leathery integument, the exoskeleton being much 

 reduced. There is a partial bilateral symmetry. The 

 madreporite is usually internal, the fluid filling the ambulacral 

 system coming from the body cavity. The external surface is 



