SUBKINGDOM 



ECHINODERMATA. 



This and the following snbkingdoni have been erected from 

 Cnvier's Radiata. The name was given by Brngieres because 

 many of the species are spine-clad. The skin is characteristical- 

 ly hardened by secretion of lime in the form of plates, as in the 

 Sea-urchin and Crinoid, or in the form of granules and spicules, 

 as in the Sea-cucumber. They are however associated by an- 

 other and more general character — the skin is almost always per- 

 forated by foramina through which the " tube-feet," communi- 

 cating with a system of water vessels (" water-vascular system ") 

 can be protruded or retracted, constituting the organs of adhesion 

 and locomotion. 



When young the members of this group show a bilateral 

 symmetry, which, in the higher Echinoids, is in some degree re- 

 tained, but which is usually concealed in the adult by the radial 

 arrangement of the parts (usually five) of the body and its in- 

 ternal organs. 



The Echinoderms are all marine. Some are rooted to the sea- 

 bottom and thus resemble the polyps, others are free, with forms 

 globular, or truly rayed and covered with prickly armor; and 

 these lead to soft elongated organisms that mimic the mollusk or 

 seem to stand at the lowest step of the articulate division. 



Unlike the Ccelenterate the Echinoderm has the digestive canal, 

 with or without an anal opening, distinct from the general cavity 

 of the body. They are the lowest animals possessing a true 

 stomach and masticating organs. The mouth is usually central 

 and sometimes surrounded by tentacles. The nervous system 

 consists of a cord forming a ring about the pharynx, with branch- 

 ing filaments. 



The subkingdom has been represented in all geologic ages. 

 The Echinoids succeed the Crinoids and Asteroids. The Crinoids 

 abounded in the Palaeozoic seas but are now reduced to a few 

 genera; the Asteroids began in the Silurian, but now the seas of 

 all latitudes swarm with them ; while the Echinoids increase to 

 tlie Cretaceous and then decline. 



