ECHINODERMS (SPINY 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- jjS 

 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 

 oursin, 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. 61. 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 soft- animal food. The food of the sea 

 urchin is mainly vegetable, and 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 Fig. 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. 



