ANATOMY OF SEA-URCHINS. 



119 



five plates are called tlie 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 moutho 

 They partly serve to re- 

 move the faecal matter, 

 but their main function 

 is not known. 



Besides the pedicel- 



J ^g^vMi ^i^^^^^W^ '&$2 larise, Loven has discov- 

 ered on most living 

 Echini, with the excep- 

 tion of Cidaris, small 

 button-like bodies called 

 sphcBridia, situated on a 



Fig. 82. View of the calcareous net-work * ,, 



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



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



zontal nei-work of straight rods.-After Gcgen- slightly marked tubercle. 



baur - They are supposed to be 



sensorial, probably organs of taste. 

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



Fig. 83.-Shell of a Sea-urchin (Str&ngylocentrotm hmdus). a, amis; oe, oesophagus, 

 i, intestine; *, one of the rods of the tooth-apparatus; m, muscles of the jaw*; ^ ^^ ves- 

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



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

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



