ECHINODERMATA. 97 



SUB-KINGDOM III ECHINODERMATA. 



CHAPTER X. 



ECHINODERMATA. 



The Echinodermata may be defined as simple marine organisins, 

 which are mostly bilaterally symmetrical when young, hut which in 

 the adult condition have this bilateral symmetry more or less exten- 

 sively masked by a radial (usually fentamerous] arrangement of 

 their parts. An alimentary canal, with or without a distinct anus, 

 and separate from the proper body-cavity is present. A system of 

 water-vessels, often communicating directly with the e.rterior, and gen- 

 erally connected with protrusible tubes ['^feet "), is present. The nervous 

 system is radiate, consisting of an oesophageal ring and radiating 

 branches. The integument is characteristically hardened by the deposi- 

 tion in it of carbonate of lime in the form of plates, granules, or .spicules. 

 The members of this sub-kingdom are popularly known as Sea- 

 urchins, Star-fishes, Brittle-stars, Feather-stars, Sea-cucumbers, &c., 

 and derive their name of Echinodermata (Gr. echinos, a. hedgehog ; 

 and derma, skin) from the generally prickly nature of their integu- 

 ments. In all, the skin is possessed of the power of secreting 

 carbonate of lime, but in very different degrees. In the Sea- 

 urchins this goes so far that the body becomes enclosed in a. kind 

 of box, composed of numerous calcareous plates firmly jointed to- 

 gether. In the Star -fishes and their allies the skin is rendered 

 prickly by grains, tubercles, or spines of calcareous matter, and the 

 body is either destitute of regular plates or is only partially enclosed 

 by them. In the Sea-cucumbers, again, the calcareous matter is only 

 present in the form of minute grains scattered in the skin. When 

 adult, they all show a more or less distinctly radiate structure, 

 which is most conspicuous in the star-shaped Star-fishes and Sand- 

 stars, but can be detected in all the members of the class. When 

 young, however, they almost always exhibit what is called " bilateral 

 symmetry '' — that is to say, they show similar parts on the two 



