386 



ZOOLOGY 



SECT. 



central. In the adult, by the intercalary development of numerous 

 additional ossicles, these primary plates of the apical system, as 

 it is termed, lose their original arrangement, and become no longer 

 recognisable. 



2. EXAMPLE OF THE ECHINOIDEA. 

 A Sea-urchin. (Strongylocentrotus or Echinus.) 



General External Features. - - The Sea-urchin (Figs. 32G 

 and 327) is globular in shape, but somewhat compressed in one 



FIG. 325. Echinus esculentus, peristome. 1, tube-feet of the lower ends of the radii; 

 2, branchia ; 3, teeth ; 4, oral tube-foot (tentacle) ; 5, peristomial membrane (From 

 MacBricle, after Kiikenthal.) 



direction, so that two poles are distinctly recognisable. At one of 

 these the degree of flattening is greater than at the other ; this is the , 

 oral pole, the opposite pole being termed the anal or aboral. At 

 the oral pole is a rounded aperture, the mouth, through which may 

 be seen projecting five hard white points, the extremities of the 

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

 the peristome or peristomial membrane (Fig. 325). At the anal pole 

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

 surrounding which is termed the periproct (Fig. 327). 



The entire surface, with the exception of the peristome and 

 periproct, is bristling with spines- cylindrical, pointed, solid appeu- 



