CHAPTEE IX 

 EOHINOBEEMATA 



Characteristics. — The echinodermata are a sharply- 

 defined, well-established group. The name refers to the 

 spiny skin, and this together with a symmetrical, radiate 

 structure characterizes all the members of the group. 

 They were once classed with the coelenterata, but the 

 groups differ materially. 



The most important characteristics are the presence 

 of a body cavity quite distinct from the digestive tract, 

 the presence of a circulatory system, and the develop- 

 ment of the complete alimentary canal with an open- 

 ing at both ends. Further, the tissues are much more 

 highly differentiated than those of the coelenterata. 

 There is a well-defined external skeleton made of hard, 

 movable plates comparable to the bones of higher ani- 

 mals, a muscular system that controls complex move- 

 ments, gills which are responsible for respiration, a 

 blood circulation confined to definite vessels, a repro- 

 ductive and a nervous system. 



As animals increase in complexity more and more 

 attention must be paid to their structure because the 

 arrangement of associated cells which determines the 

 peculiar form of the animal, determines also the pecu- 

 liar way in which the physiological characteristics of the 



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