640 ECH1NODERMATA 



urchins, fit immovably together and extend from the peristome on the 

 oral side to the apical plates on the aboral side of the body. Two of 

 these rows are situated in each radius (1) and two in each interradius 

 (2), the radial plates, as we have just seen, being pierced by the 

 ambulacral pores. The plates of the corona bear the tubercles (6) on 

 which the spines articulate. 



The peristome is sometimes without plates and entirely membranous ; 

 in some forms small irregular plates are imbedded in it; in some it con- 

 tains rows of plates which are continuous with those of the corona. 

 The apical plates occupy the center of the aboral surface. In the regu- 

 lar sea-urchins (Fig. 987) they consist of the anal plates which surround 

 the anus (5) and form the periproct, the five genital plates (4) at the 

 aboral ends of the five interradii, in each of which is a genital pore, and 

 five radial plates at the aboral ends of the five radii. Each of the latter 

 is pierced by a pore through which the terminal end of the radial canal 

 projects; inasmuch as it is pigmented, it was formerly supposed to be 

 an eye, and the plate is generally called the ocular plate. In the regular 

 sea-urchins one of the genital plates is the madreporite (3). In the 

 other sea-urchins the anal plates are not in the apical system and the 

 arrangement of the other plates is changed, the madreporite being 

 usually in the center of the group (Fig. 990,3). 



The ambulacral system consists of a ring canal (Fig. 966, B, 4) 

 around the oesophagus and five radial canals (7) which pass along the 

 radii on the inner surface of the shell to the radial plates, through which 

 they project. The branches going to the feet possess each an ampulla. 

 The stone canal (5) passes from the ring canal to the madreporite and 

 is surrounded by the axial sinus with its axial organ. 



The alimentary canal consists of a wide tube (6) which, beginning 

 at the mouth, winds completely around the inner wall of the body cavity, 

 then, turning on itself, winds around again in the opposite direction and 

 ends at the anus. An accessory gut branches off from the intestine near 

 the mouth and, running along parallel with it, joins it near its middle. 

 In most sea-urchins 'a complicated dentary apparatus (12), sometimes 

 called the lantern of Aristotle, surrounds the ossophagus. It is com- 

 posed of five groups of calcareous plates bound together by muscles, and 

 from it five calcareous teeth project through the mouth to the outside. 



Respiration and excretion are performed by the entire surface of 

 the body, and especially by the ambulacral appendages, the peristomal 

 gills, and perhaps the accessory intestine. The superficial oral nervous 

 system is intra-skeletal, lying along the ambulacral vessels. Special 

 sense organs are poorly developed. SphaBridia are almost universally 

 present. In some genera pigment eyes are distributed over the body. 



