ANATOMY OF SEA-UBCHINS. 



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- 

 lariffi are three-pronged, 

 knob-like spines, scat- 

 tered over the body, es- 

 pecially near the mouth. 

 They partly serve to re- 

 move the fffical matter, 

 but their main function 

 is not known. 



Besides the pedicel- 

 lariffl, Loven has discov- 

 ered on most living 

 Echini, with the excep- 

 tion of Oidaris, small 

 button-like bodies called 

 sphmridia, situated on a 

 short stalk, moving on a 

 slightly marked tubercle. 

 They are supposed to be 

 sensorial, probably organs of taste. 

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



h — 



View of the calcareous net-work 

 from a plate of the integument of a Sea-urchin 

 {CidaHs). b, section perpendicular to the hori- 

 zontal net-work of straight rods.— After Gegen- 

 bdur. 



Fig ^ —S\ie\\ oi ■d.Sed.-w-iQhm {Strongyloeentrotns lividus). «, anue; oe, cesophagus; 

 L intestine; «, one of thu rods of the tooth-apparatus; m, 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 £wo halves, oral and aboral. Remov- 

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



