388 



ZOOLOGY 



SECT. 



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

 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 



FIG. 308. Strongylocentrotus, entire animal with the tube-feet extended. (From Brehm'i 



Thierleben.) 



the plates of the corona are in twenty regular meridional 

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

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

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

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

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

 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. 



