ANATOMT OF SEA-URGHmS. 



119 



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D 





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

 five smaller plates at the end of each ambulacral series is an 

 J eye-speck. The pedieel- 



\ _ _ __-_ larise are three-pronged, 



knob-like spines, scat- 

 tered over the body, es- 

 pecially near the mouth. 

 They partly serve to re- 

 move the fseeal matter, 

 but their main function 

 is not known. 



Besides the pedicel- 

 larise, Lovon has discov- 

 ered on most living 

 Echini, with the excep- 

 tion of Cidaris, small 

 button-like bodies called 

 sphwridia, 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 







>. 





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Fig. 82.— View of the calcareous net-work 

 from a plate of the integument of a Sea-urchin 

 {Cidans). b, section peroeuchcular to the hori- 

 zontal net-work of straignt 

 baur. 



; rodu.— After Gegen- 



Pig. 88.— Shell of a Sea-urchin (Strongylocentrohm lividus). a, anus; oe, (esophagus; 

 i, intestine; a, one of the rods of the lobth-apparatus; 7n, 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 two halves, oral and aboral. Eemov- 

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



