514 



ECHINODERMATA ECHINOIDEA 



peristome extends outwards beyond the water -vascular ring, 

 whereas in Asteroidea it is contained entirely within the 

 ring. In the primitive Cidaridae (Fig. 235) the whole 

 peristome down to the lip surrounding the mouth is covered 

 with a series of ambulacral and interambulacral plates similar to 

 those forming the corona, though smaller and not immovably 

 united, and the series of tube-feet is continued on to it. It is 



, tooth 



inter 



amb 



Fig. 230. The dried peristome of Echinus esculentus and the surrounding portions of 

 the corona, xl. amb, Ambulacral plate; b.t, buccal tube -foot; g, gill -cleft; 

 inter, interambulacrum ; per, peristome. 



thus evident that the peristome is merely part of the corona, 

 which has become movable so as to permit of the extension of 

 the teeth. In Echinus the peristome is continued in each 

 interradius into two branched outgrowths called gills, the relation 

 of which to the respiratory function will be described later. 

 These gills (Fig. 229, 2) are situated in indentations of the edge 

 of the corona called " gill-clefts " (Fig. 230, g). 



The most conspicuous plates in the peristome are those 





