164 OBGANS OF DIGESTION. 



the urchins the mouth is for the most part armed with jaws 

 and teeth, and the oral and anal openings, gradually becoming 

 more separate, occupy distinct positions on the shell; in. Echinus 

 and Cida?'is, the mouth is found at the under pole, and the 

 anus at the upper pole of their globular shells. Fig. 1 74 shows 

 the structure of a common urchin (Echinus esculentus) ; the test 



Fig, 174. The anatomy of the Echinus esculentus. 

 is divided near its equator, and the small section is raised 

 to shew the mouth from above ; k, h is the lantern, with the 

 pyramids and teeth ; the esophagus (m) is long and delicate, 

 and continuous with the stomach () ; the first convolution 

 of the intestine is seen at o, and the second at q, r\ the 

 rectum (s) terminates in the centre of the opening formed by 

 the circle of ovarial plates, and surrounded by the branching 



