ECHINODERMATA — SEA URCHINS 169 



with the digestive system. It consists principally of five 

 enameled teeth meeting in a point ; they project slightly out 

 of the mouth, and are moved by a complicated set of muscles. 

 The teeth and muscles are supported by five pyramidal plates 

 which with their connections (epiphyses, bases and compasses) 

 form the skeletal part of Aristotle's lantern. These teeth are 

 used to grind the food into bits. A wide digestive tube extends 

 from the mouth in the center of the ventral (lower) surface to 

 the anus in the center of the dorsal (upper) surface, winding 

 around the inside of the test. There is no such differentiated 

 stomach, nor radial divisions of the digestive system, as occur 

 in the starfish. 



The five double rows of long, slender tube-feet enable the 

 animal to cling to the rocks over which it slowly glides in search 

 of the algae and small organisms on which it lives. Sea urchins 

 are usually vegetable feeders. 



The hlood vascular circulation is very similar to that of the 

 starfish, having in addition two large intestinal veins parallel 

 to the intestine. The blood has about the same composition 

 as that of the starfish. 



Respiration is effected mainly by the upper end of the digestive 

 canal and by the shrub-like gills at the ventral margin of the test. 



The nervous system is similar to that of the starfish, and there 

 are present, likewise, five eye spots at the tips of the ambulacra 

 in the ocular plates. 



At the tip of each interambulacrum is a genital or basal 

 plate in which are openings for the extrusion of the repro- 

 ductive elements. Through these pores the eggs and sper- 

 matozoa are cast out into the water during summer, where 

 after union they rapidly develop into tiny translucent bi- 

 lateral larvae. These larvae swim about and feed on smaller 

 creatures for several weeks, finally developing a minute, globu- 

 lar, radial sea urchin at the posterior end. In a few hours 

 the larval portion becomes absorbed and leaves the minute sea 

 urchin to drop to the sea bottom. 



