ECHINODERxMATA 641 



448) is composed of twenty meridional rows of plates, two in each 

 radius (ambulacrum), and two in each interradius. The plates of the 

 ambulacra are distinguished by the presence on them of tfie pores 

 for the tube feet. These pores are in pairs, since each ampulla com- 

 municates with its tube feet by two canals. Thus water can circulate 

 in and out of the tube feet and respiration is facilitated. At the aboral 

 pole each radius ends in a single ocular plate ^ which bears the opening 

 of the terminal tentacle, and each interradius in 2i genital plate which 

 abuts upon the periproct and bears the opening of a gonoduct. One 

 of the genital plates bears also the madreporite. All the plates are 

 studded with bosses of various sizes, to which articulate the concave 



amh. 







:/2'^ 



-ijill 







^crist. 



Fig. 447. Echinus rniliaris from the oral side. amb. ambulacrum; gill, gill; 

 inter, interambulacrum ; perist. peristome. 



bases of the large and small spines and the pedicellariae. The spines, 

 unlike those of starfishes and brittle stars, which are moved with the 

 ossicles under them, have muscles of their own. These are in two sets, 

 an outer one which causes movements, and an inner " catch" muscle 

 (p. 143) which holds the spine firmly in position. On level ground the 

 spines take part at times in locomotion, the animal using them like 

 stilts. The pedicellariae (Fig. 449), which have three jaws, are of 

 several kinds. Gemmiform pedicellariae have stiff stalks and globular 

 heads with a poison gland in each jaw. The tridactyle kind have a 

 flexible stalk and long jaws. The ophiocephalous kind are smaller and 

 have a flexible stalk and broad, toothed jaws. The trifoliate pedicellariae 



