286 



THE ECHINOIDEA 



the presence of five sharp teeth, which, if not extruded so as to be 

 seen, can at once be felt. At the other end of the axis of the 



body there is a smaller membranous 

 area, the " periproct," in the centre 

 of which is the anus. The periproct 

 is surrounded by a circle of five plates 

 (Fig. I,), one of which is larger than 

 the rest ; this large plate bears a 

 small perforated prominence, called 

 the " madreporiform tubercle " or 

 " madreporite " (m). Through the 

 no. i. pores in this plate water passes to 



csvdentus Linn apical the series of canals known as the 

 e^^ water-vascular system. Each of the 



tai pore ; o, ocular plates ; an, anal other four plates of this apical circle 



plates covering the periproct, in the . r . j , < ^r 



centre of which opens the anus, is perforated by one of the pores 



through which the genital products 



escape to the exterior. These five plates are therefore known as 

 the " genitals " (g). Alternating with them are five smaller plates 

 (o), each perforated by a pore, through which passes a process 

 ending in a sensory eye-spot. These plates are therefore called 

 the " oculars." The whole ten plates form a group known as the 

 " apical system." Between the apical system and the peristome is 

 the main test. Ten lines of suckers may be seen in a fresh 

 specimen, radiating from the five ocular plates ; two lines start 

 from each ocular, and pass in a straight series round the test to 

 the edge of the peristome. As the two series of suckers are 

 arranged like the rows of trees in an avenue, the area bounded by 



Fio. II. 



Ambulacral (1) and interam- 

 bulacral (2) plates of Echinus 

 uculentus. fl, fl are the primary 

 and secondary tubercles ; g, 

 miliary granules ; p, primary 

 ambulacral plate ; d, epipodium 

 with a pore pair; b, boss; m, 

 mamelon. 



them has been called an ambulacrum. 1 As there is one ambula- 

 crum to each ocular, there are five in the complete test, separated 

 by five broader interambulacra. 



Each ambulacrum and interambulacrum consists of a double 

 row of plates in vertical series running from the apical system to 

 the peristome. Each interambulacral plate (Fig. II. 2) is irregu- 

 larly pentagonal in form ; the angles are sharp and regular, but 

 the plate is elongated in a horizontal direction. Each plate bears 

 a number of " tubercles," of which there are three sizes primary 



1 From ambulacrum, an avenue or a walk between trees. 



