360 



ESSENTIALS OF ZOOLOGY 



lacral rows. These rows correspond in position to the ambulacral 

 grooves of the starfish. 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 



Madreportfe 



7 



p/afes 



ppp^ 



Afrfbu/crcraf 

 p/afes 



Fig. 133. — Dried test of the sea urchin, Arbacia. A, Shows arrangement of the 

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

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



removing algae from rocks for food. This arrangement constitutes 

 Aristotle's lantern, and the esophagus leads internally from its 

 aboral part. The principal organs of respiration are the interradial 

 pouches and the tube feet. The nervous system is composed of a 

 circumoral ring with radial cords extending into the ambulacral 



