ANATOMY OF SEA-URCHINS. 



201 



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- 

 larise 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 that of touch. 



Besides the pedicel- 

 lariae, Loven has discov- 

 ered on most living 

 Echini, with the excep- 

 tion of Cidaris, small 

 button-like bodies called 

 , sphcericlia, situated on a 



Fig. 14ft. View of the calcareous nftt->vork * 



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

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



zontal net-work of straight rodu. -After Gegen- slightly marked tubercle 



They are supposed to be 



sensorial, probably organs of taste and smell.* 

 The internal anatomyof the sea-urchin may be best studied 



Fig. 146. Shell of a Sea-urchin (S/rongylocentrofns lividug). a. anus; oe, oesophagus; 

 i, intestine; *, one of the rods of the tooth-apparatus; m, muscles of the jawf ; p, ves- 

 eels of the sucking feet; }X>. extremity of the water- vessel ; <xt, ocular plate; f, ovary. 



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



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



' * In the iutcrambulacral spaces are blue spots, viz., compound eyes. 



