AJ^ATOMF OF SEA-UBCHINS. 



119 



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

 fiye smaller plates at the end of each ambulacra] series is an 



eye-speck. The pedicel- 

 lariffi are three-pronged, 

 knob-like spines, scat- 

 tered over the body, es- 

 pecially near the mouth. 

 They partly serve to re- 

 move the fsecal 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 

 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.— Vifw of the calcareous net-work 

 fi-om a plate of the Integiiment of a Sea-urchin 

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

 zontal net-work of Btruignt rods After Gegen- 



baur. 



Fig. 83.— Shell of a Sea-urchin {Slrongylocentrot.iin lividus). a, anue; oe, cfisophagns; 

 i, intestine; «, one of the rods of the Looth-apparatus; 'm, inusclea of the jaws; p, ves- 

 sels of the sucking feet; 7;o, 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. 



