ANATOMY OF SEA-URCHINS. 



119 



five plates are called the genital plates, while in each of the 

 live smaller plates at the end of each ambulacra! series is an 



eye-speck. The pedicel- 

 larias are three-pronged, 

 knob-like spines, scat- 

 tered over the body, es- 

 pecially near the mouth. 

 They partly serve to re- 



& HI.':' ? ; 1 14 move the fo?cal matter, 



but their main function 

 is not known. 



Besides the pedicel- 

 larise, Loven has discov- 

 ered on most living 

 Echini, with the excep- 

 tion of Cidaris, small 

 button-like bodies called 

 sphceridia, situated on a 



Fig. 88. View of the calcareous net-work - 11 



from a plate of the integument of a Sea-urchin short Stalk, moving Oil a 



(Cidaris). b, section perpendicular to 1 he hori- 1-1,1 i i j. i i 



zontal net-work of straight rods. -After Gegeii- slightly marked tubercle. 



They are supposed to be 

 sensorial, probably organs of taste. 



The internal anatomy of the sea-urchin may be best studied 



Fig. &3. Shell of a Sea-urchin (Strongylocentrotita lividus). a, anns; oe, oesophagus; 

 i, intestine; s, one of the rods of the tooth-apparatus; , muscles of the jaws; p, ves- 

 sels of the sucking feet; po, extremity of the water-vessel; ca, ocular plate; v, ovary. 



by cutting the shell into two halves, oral and aboral. Eemov- 

 ing the aboral end, the digestive canal may be seen in place. 



