PHYLUM ECHINODERMATA 



221 



removing algae from rocks for food. This arrangement constitutes 

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

 aboral part. Aborally, each ambulacral area ends in a single ocular 

 plate at the periproct. A series of genital plates interspace the 

 ocular plates of the periproct, and one of these genital plates has 

 become the madreporite. The anus is a small pore located near the 

 center of the periproct. 



Jtomac/? 



flmbalacral 

 Ridge 



Fig. 118. — A portion of the test of the sea urchin has been removed to show the 



internal organs. 



Upon first view, a dissected sea urchin seems to be almost entirely 

 gonads, internally. In each pentagonal section there is a mass of 

 the gonadal structure which is held in place by a band of tissue 

 known as the genital rachis extending from the aboral wall. Small 

 genital ducts lead from each gonad through a pore in its adjacent 

 genital plate. The general arrangement internally includes a water 

 vascular system composed of a stone canal leading from the mad- 

 reporite to a circumoral or circumesophageal canal which encircles 

 the mouth. A radial vessel extends from the circumoral canal through 

 the length of each ambulacral arch. In each ambulacral plate are 

 found two pores for a tube foot which is a lateral branch of the 



