STARFISH, SEA URCHIN, BRITTLE STARS 101 



mentary canal is occasionally ejected from the body and 

 in some cases, at least, becomes completely renewed. 



Economic importance of the echinoderms. — In the past 

 starfish have caused much annoyance to oystermen because 

 of the large numbers of oysters devoured by these echino- 

 derms for food. Of late years, however, the oystermen have 

 learned how to keep the oyster beds free, in a measure, 

 from these pests. 



Certain species of sea cucumbers are used as food by the 

 Chinese. Great numbers of them are caught on the coral 

 reefs of the Pacific Ocean and China Sea and sold iu Chinese 

 ports imder the trade name of " trepang." 



Relationships and characteristics of the echinoderms. — 

 The echinoderms stand almost alone and without any near 

 relatives. They all have an alimentary canal separate from 

 the body cavity and a well-developed nervous system which 

 place them above all animals so far studied. Without 

 exception they live in the sea, and in all, the parts of the 

 body are radially arranged. Moreover, the parts occur to 

 the number of five or are repeated in multiples of five. 

 This characteristic is well shown by the common starfish 

 which has five arms and by the species of starfish that 

 has thirty arms. Again, the members of this Isranch 

 possess a water-vascular system and this is eminently 

 characteristic, for no other liranch of animals is found 

 with this structure. Unhke the ccelenterates, the echino- 

 derms show no tendency to l)ud and form colonies of zooids 

 and the gi-eat majority of them are free. They also 

 possess an exoskeleton, which, in some, consists of plates 

 joined together continuously and in others of scattered 

 plates imbedded in the skin. In many the body is armed 

 with spines, hence the name of the branch. 



