CHAPTEK V 

 PHYLUM IV. : ECHINODERMATA 



(STARFISHES, SEA-URCHINS, ETC.) 



THE Echinoderms are multicellular animals, distinguished 

 from the Coelenterates by the presence in them of a distinct 

 body-cavity or coelom lying outside the digestive or alimentary 

 canal, and containing the vital organs of the body. 



They are usually five-rayed and radially symmetrical 

 forms, with a skin hardened by small calcareous plates and 

 spines. A unique character, found only in this phylum, is 

 the presence of a peculiar water-vascular system, by means 

 of which water can be taken into the body and pumped into 

 the " tube-feet," which in many of them are the chief organs 

 of locomotion. There is, however, no well-defined blood- 

 vascular or excretory system. The phylum is large and 

 varied, and is divided into five classes, all of which are 

 marine. 



Class I. : ASTEROIDEA 

 Type : The Common Starfish (Asterias rubens). 



This Starfish is well known on our coasts, and can be easily 

 recognised by its five thick rays or arms, covered with little 

 limy plates bearing small spines ; one specially well-marked 

 line of spines runs down the centre of the upper surface of 

 each arm, thus distinguishing it from another species (Asterias 

 glacialis), which has less numerous and larger spines arranged 

 in three to five special lines down the arms. When lifted up, 

 the arms of the common starfish droop limply, a character 



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