PHYLUM ECHINODEKMA 



Sub-Phyla 



1. Eleutherozoa 



2. Pelmatozoa 



The animals included in this division are all marine ; they 

 comprise the star-fishes, brittle-stars, sea-urchins, sea-lilies, 

 sea-cucumbers, and the extinct blastoids and cystideans. 

 The body is very often radially symmetrical, the symmetry 

 being generally pentamerous. But in many cases there is 

 also a more or less well-marked bilateral arrangement of 

 parts. The echinoderms are never compound animals. In 

 the majority of cases the alimentary canal terminates in 

 an anus. A body-cavity or coelom is present and surrounds 

 the alimentary canal. The water- vascular system (fig. 43) 

 is one of the distinguishing features of the group : it con- 

 sists of a set of vessels containing a watery fluid and 

 generally placed in communication with the sea-water by 

 means of a canal ; one vessel forms a ring round the 

 oesophagus from which radiating trunks are given off. 

 The water-vascular system functions in respiration and 

 as a sensory organ, and generally also in locomotion. A 

 nervous system is present ; one part of it has a distribution 



