SUB-KINGDOM IV. ECHINODERMATA. 



Classes. Orders. 



THE animals included in this sub-kingdom are all 

 marine; they comprise the starfishes, brittle-stars, sea- 

 urchins, sea-lilies, sea-cucumbers, and the extinct blas- 

 toids and eystideans. The body is very often radially 

 symmetrical, the symmetry being generally pentamerous. 

 But in many cases there is a more or less well-marked 

 bilateral arrangement of parts. Unlike so many of the 

 coelenterates the echinoderms are never colonial forms. 

 In the majority of cases the alimentary cavity terminates 

 in an anus. A distinct vascular system is present, and 

 also a set of vessels containing a watery fluid, known as 

 the ivater-vascular system. This consists of a ring round 

 the ossophagus from which five radiating trunks are given 

 off, and it functions generally in locomotion, and probably 



