ECHINODERMA. 



This term means spiny skin, and both starfishes and sea- 

 urchins possess this peculiarity in a high degree. But 

 besides this external characteristic there are many other 

 features which distinguish the 

 group. In fact, there is scarcely 

 a division in the whole animal 

 kingdom more sharply marked 

 off from other forms than this. 

 In all the body is built on that 

 radiate plan which is so promi- 

 nent in starfish and urchin, 

 and in all except a few starfish 

 there are five rays, although in 

 some the rays may subdivide. 

 This radiate condition affects 

 not only the external surface, 

 but may extend to every sys- /Y \\ 



tern as well. And vet we may 



J J FIG. 139. Larva of a starfish, en- 



trace in every form a bilateral- larged. m, mouth ; v, vent, 

 ity, and development shows that the bilateral condition is 

 primitive, the radial character being acquired with growth. 

 Another characteristic is the ambulacral apparatus with its 

 water- vascular system, and a third feature is the possession 

 of a large body-cavity distinct from the alimentary canal. 

 In all there is the formation of calcareous plates in the 

 skin, and in all except the Holothurians these plates are 

 developed into a more or less solid skeleton. All possess a 



