The Animal Kingdom - 661 



Fig. 32-33. Echinoderms, rep- 

 resenting the five classes. A, a 

 sea urchin, Class Echinoidea; 



B, a sea lilly, Class Crinoidea; 



C, a sea cucumber, Class Holo- 

 thuroidea; D, a brittle star, 

 Class Ophiuroidea; and E, a 

 common starfish, Class Asteroi- 

 dea. (From General Zoology, 

 by Miller and Haub. Holt, 

 Rinehart and Winston, Inc.) 



Embedded in the body wall there is an endo- 

 skeleton, consisting of a number of hard 

 calcareous plates, the ossicles; and projecting 

 outward from the body there are many cal- 

 careous spines. Collectively the ossicles and 

 spines, plus a system of interconnecting mus- 

 cles and strands of connective tissue, give 

 great strength to the body wall. 



Usually the ossicles are about the size of 

 very small pebbles, as in the common starfish, 

 or they may be of microscopic size, as in the 

 sea cucumbers. The spines also display con- 

 siderable variation. Common sea urchins 

 (Fig. 32-33) have strong pronglike spines, 

 usually somewhat less than an inch long; but 

 one semitropical genus (Diadema) has sharply 



pointed poison spines that may extend more 

 than nine inches out from the surface of the 

 body. On the other hand, the spines of many 

 echinoderms are slender and delicate, as in 

 the sea cucumber (Fig. 32-33). 



The symmetry of the echinoderms is 

 another unique identifying feature. Super- 

 ficially the adults display radial symmetry, 

 a development that probably is related to 

 the sedentary history of the group. The body 

 always possesses a central part, the central 

 disc, where the digestive tract is found. And 

 surrounding the central disc there are five 

 radially arranged body sections from which, 

 in many species, more or less distinct arms 

 (rays) project (Fig. 32-34). 



