488 THE ANATOMY OF INVERTEBRATED ANIMALS. 



the periproct. Each of these is perforated, and supports the 

 eye-spot; it is thence called an ocular plate. The apical ex- 

 tremities of the interarabulacra, on the other hand, correspond 



Fro. 142. (After Mliller.) A, three ambulacra! plates of Echinus sphcera, exhibit- 

 ing the eutures of the pore-plates of which each ambulacral plate is composed. B, 

 part of the- petaloid ambulacrum of a Clypeastroid. 



with the five larger plates, which alternate with the ocular 

 plates, and, like them, are perforated. The aperture is, how- 

 ever, larger, and constitutes the exit for the generative prod- 

 ucts. One of these five genital plates is larger than the 

 others, and presents a peculiar porous convex surface, which 

 is the madreporic tubercle or madreporite. The latter is 

 therefore interambulacral in position, as in the Star-fish. 



Comparison with the elongated Echinoderms shows that 

 the madreporite lies in the right anterior interradius of the 

 sea-urchin, so that the anterior ambulacrum is that which lies 

 to the left of the madreporite, when the latter is directed for- 

 ward. In consequencs of being able to distinguish this odd 

 or anterior radius, it is possible in any of the Echinidea to 

 separate the three anterior ambulacra, as the trivium, from 

 the two posterior, the bivium ; and in the fossil genus Dy- 

 saster, this separation of the ambulacra into trivium and bivi- 

 um exists naturally. Mliller has pointed out that in all the 

 flattened Echinidea, with a special ambulatory surface, the 

 latter is formed by the bivial ambulacra and interambulacra, 

 while, in the similarly modified Holothuridea, the animal rests 

 upon the trivium. 



Within the circle formed by the genital and ocular plates 

 the periproct presents a variable number of calcifications, of 

 which one, the anal plate, is larger than the rest. The anus 

 lies excentrically, between this plate and the posterior margin 

 of the periproct. 



With the exception of certain palaeozoic forms (Palcechi- 



