88 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BuU. 



arms, firmly fitted together. Thus the entire aboral surface of 

 both disk and arms of the starfish can be represented in the 

 sea-urchin only by the few plates of the periproct. 



Each ambulacral area ends near the periproct in a single 

 plate called the ocular plate (Fig. 13), perforated by a pair of 

 minute openings from which a tentacle protrudes, as at the end 

 of the arm of the starfish. The interambulacral areas likewise 

 terminate in single large plates called the genital plates, because 

 they bear the five openings of the genital glands (Plate XXV). 

 One of these genital plates is larger than the others, and is per- 

 forated by numerous minute pores which lead to the water- 

 vascular system. This plate is therefore termed the madreporic 

 plate (Fig. 13). 



Fig. 13. Arbacia punctulata. Out- 

 line of plates of aboral surface, 

 showing the four plates of the peri- 

 proct, surrounded by the five ocular 

 and five genital plates at the ends 

 of the ambulacra! (i?) and inter- 

 ambulacral (/) zones, respectively; the 

 genital plate marked M is the madre- 

 poric plate; T, tubercles for attach- 

 ment of spines ; P, pores for the tube- 

 feet 



At the margin of the peristome are five pairs of much 

 branched processes, having a respiratory function, and termed the 

 dermal branchiae, or gills. 



Just inside the test at the border of the peristome, and resting 

 upon the bases of the ambulacral areas, are five pairs of calcareous 



