ANATOMY OF SEA-URCHINS. 



119 



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

 five smaller plates at the end of each ambulacral 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- 

 move the faecal 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 

 sphcsridia, situated on a 

 short stalk, moving on a 

 slightly marked tubercle. 

 They are supposed to be 

 sensorial, probably organs of taste and smell. 



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



Fig. 82. View of the calcareous net-work 

 from a plate of the integument of a Sea-urchin 

 (Cidaris). b, section perpendicular to the hori- 

 zontal net-work of straight rods. After Gegeu- 

 baur. 



Fig. as. Shell of a Sea-urchin (Strongytocentroftt* lividus). a, anus; oe, oesophagus; 

 i, intestine; . one of the rods of the tooth-apparatus; m, muscles of the jaws; />, ves- 

 eels 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. Remov-. 

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



