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TEXTBOOK OF ZOOLOGY 



of plates can be compared to a starfish with its arms turned up over 

 its body until the tips all touch each other. The surface of the 

 skeleton bears processes which support movable spines. Tube feet 

 may be thrust out through perforations in the plates of the ambu- 

 lacral rows. These rows correspond in position to the ambulacral 

 grooves of the starfish. The plates of the inter-ambulacral rows 



Madrepori'fe 



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9^ 



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 p/afes *^ 



Anybu/cfcraf 

 p/afes 



Fig. 117. — Dried test of the sea urchin, Arhac'xa. A, shows arrangement of the 

 plates on the aboral side; B, oral view showing mouth and perioral area. (From 

 Wolcott, Animal Biology, published by McGraw-Hill Book Company, Inc.) 



are not perforated. The mouth of this type of animal is located 

 ventrally (orally), and it is guarded by five projecting skeletal 

 processes called teeth. These converge over the aperture and are 

 set in a skeletal case which is composed of many hard ossicles and 

 contains the muscles for moving the teeth. The teeth are used in 



