ECHINUS PERIPROCT AND PERISTOME 



SI3 



series is termed the " genital plate," because it is pierced by the 

 duct of one of the five genital organs. One of the genital plates 

 is also pierced by the madreporic pores. Some zoologists have 

 separated the ocular and the genital plates under the name of 

 " calyx " from the rest of the corona, under a mistaken idea that 

 they are homologous with the plates of the body or calyx of a 

 Crinoid. 



The periproct (Fig. 228, 4) is covered with small plates 



Fig. 229. — The peristome of Echinus escwlentits. x 2. 1, Tube-feet of the lower 

 ends of the radii ; 2, gill ; 3, teeth ; 4, bueoal tube-foot ; 5, smooth peristomial 

 membrane. (After Kiikenthal.) 



and bears a few pedicellariae. The peristome (Fig. 229) is 

 covered by flexible skin with abundant pedicellariae ; it termin- 

 ates in a thick lip surrounding the mouth, from which the tips 

 of five white teeth are just seen projecting. There are ten short 

 tube-feet projecting from the peristome — one pair in each radius 

 — and each tube-foot terminates in an oval disc and is capable 

 of little extension, and each has around its base a little plate. 

 The presence of these tube- feet shows that in Echinus the 

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