388 



ZOOLOGY 



SECT. 



by sutures so as to form a continuous shell or corona. Asthenosoma, 

 a deep-sea genus, differs from all the rest in having a corona 

 possessing a certain degree of flexibility, and performing move- 

 ments which are brought about by the contractions of five 

 longitudinal bands of muscle running along the ambulacral areas 

 on the inner surface. 



In the globular forms, or regular Sea-urchins, the mouth is situ- 

 ated at the ventral pole of the globe, the anus at the dorsal, and 



Fu;. 308. Strongylocentrotus. entire animal with the tube-feet extended. (From Brehm's 



Thierleben.) 



the plates of the corona are in twenty regular meridional rows, 

 arranged in ten zones, five ambulacral and five inter-ambulacral, 

 as described in the account of Echinus, with peristome, periproct, 

 ocular and genital plates, and madreporite. Spines (Fig. 309), 

 pedicellarice (Fig. 310), and sphwridia are present, as already 

 described (p. 363), the last-named appendages, however, being 

 absent in one group. The spines are usually defensive organs 

 simply, but in some Sea-urchins they act also as the locomotive 

 organs, the animal moving by their agency along the sea-bottom. 



