ECHINODERMS (SPLVY ANIMALS) 



39 



feet arranged? Where do the rows begin and end? Would you 

 think a sea urchin placed upside down in water, could right itself 

 less or more readily than a star- 

 fish ? What advantage in turn- 

 ing would each have that the 

 other would not have? The 

 name sea urchin has no refer- 

 ence to a mischievous boy, but 

 means sea hedgehog (French 

 oiitsin, hedgehog), the name 

 being suggested by its spines. 

 Comparison of Starfish and 

 Sea Urchin. — The water sys- 

 tem of the sea urchin, consist- 

 ing of madreporite, tubes, and 

 water bulbs, or ampullae, is 

 similar to that of the starfish. 



Fig.6i. — A Sea Urchin crawling up 

 the glass front wall of an aquarium 

 (showing mouth spines and tube feet). 



The tube feet and locomotion are alike. There is no need for 

 well-developed respiratory organs in either animal, as the whole 

 body, inside and out, is bathed in water. The method of repro- 

 duction is the same. 



The starfish eats animal food. The food of the sea urchin is 

 almost exclusively vegetable, hence it needs teeth (Fig. 62, 63) ; 



Fig. 62. — A Sea Urchin 

 with spines removed, 

 the limy plates showing 

 the knobs on which the 

 spines grew. 



Fig. 63. — Section of Sea Urchin 

 with soft parts removed, showing the 

 jaws which bear the teeth protruding 

 in Fie. 62. 



its food tube is longer than that of a starfish, just as the food tube 

 of a sheep, whose food digests slowly, is much longer than that of 

 a dog. 



