Echinodermata. 341 



The food is varied. Sea urchins sometimes eat sea- 

 weeds. At other times they are found eating fish, etc., 

 that have been thrown out as refuse by the fishermen. 

 Sea urchins are of no special economic importance, neither 

 being of any use nor doing any damage. 



Respiration in the Sea Urchin. Around the mouth, on 

 the leathery membrane known as the peristome, are some 

 specially modified tube feet which are supposed to act as 

 gills. But, as in the starfish, the whole body, inside and 

 outside, is so thoroughly bathed in water that there hardly 

 seems a need of any special organs of respiration. 



The Nervous System of the Sea Urchin. This, too, is 

 very similar to what we have seen in the starfish. There 

 is a nerve ring around the gullet, from which a radial nerve 

 passes along the ambulacral line, between the rows of tube 

 feet, just within the body wall. 



At the end of the series of ambulacral plates is a single 

 plate, known as the ocular plate, on which is an eye, at the 

 very tip of each radial nerve. 



There are also, over the surface of the body, a number 

 of small spherical bodies, borne on movable stalks. These 

 bodies (spheridia) have ganglion cells in them, and are 

 regarded as sense organs. 



Development of the Sea Urchin. The ovaries are situ- 

 ated in the aboral part of the body cavity and are placed 

 in the interambulacral spaces. A duct from each ovary 

 opens through a plate at the end of the series of interam- 

 bulacral plates. These plates with the genital pores are 

 called the " genital plates," and alternate with the ocular 

 plates that have been described as occurring at the apex of 

 the series of ambulacral plates. In the male the sperma- 

 ries occupy a similar position, and have similar genital 

 pores. In the Southern sea urchin the ovaries are red 



