394 



ZOOLOGY 



teeth, surrounding the mouth is a thin, soft membrane known as 

 the jxristome or peristomial menvirane (Fig. 320). At the anal pole 

 is a much smaller aperture, the amis, the space immediately 

 surrounding which is termed the periproct (Fig. 322). 



The entire surface, with the exception of the peristome and 

 periproct, is bristling with spines — cylindrical, jjointed, solid ap- 

 pendages, the surface of which is longitudinally fluted. These are 

 movably articulated with the body so that they may be turned 

 about in all directions. When one of them is removed (see Fig. 

 338, p. 422), it is found that the joint is of the character of a ball 



Fig. 320.— Echinus esculentus, peristome. 1, tube-feet of the lower ends of the radii ; 2, 

 branchia ; 3, teeth ; 4, buccal tube-foot ; 5, peristomial membrane. {From MacBride, after 

 KUkenthal.) 



and socket, a concavity on the base of the spine fitting over a 

 hemispherical elevation on the surface of the Sea-urchin, and the 

 spine being retained in place and caused to move by means of a 

 capsule of muscular fibres enclosing the joint. Around the bases 

 of the large spines are a number of very small spinules. Here and 

 there among the spines are to be ojjserved minute pedicellaricc 

 (see Fig. 340, p. 423), which are comparable to the stalked 

 pedicellarise of Asterias ; but each has three jaws instead 

 of two, and a relatively long stalk, which is supported by a 

 slender calcareous rod. Here and there are to be found also small 



