ARBACIA PUNCTULATA 193 



of rows composed of adambulacral plates represent the interradii. 

 Meridional zig-zag lines run in each radius and interradius 

 marking the articulation seams of the interlocking plates. Sur- 

 rounding the periproct are five large almost triangular genital 

 plates. They represent the terminal plates of the interradii and 

 each shows a large genital pore. One of these plates is somewhat 

 larger than the others and has a rough surface. It is the madre- 

 poric plate and marks the plane of bilateral symmetry. Between 

 the genital plates are five small ocular plates. These belong to 

 the radii and each has two pores for the terminal tentacle. The 

 term ocular is a misnomer, for the tentacles correspond to 

 the terminal tentacles of other echinoderms and do not have 

 the structure of eyes. 



Returning to the meridional rows of plates we observe first 

 of all that the radii are much narrower than the interradii. 

 Moreover, the radial plates increase gradually in size from the 

 ocular plate to the edge of the peristome where the radius is 

 widest. On the other hand, the interradial plates, also gradually 

 increasing in size, are largest at the equator beyond which they 

 again diminish. As a consequence the radial and interradial 

 plates bordering the peristome are about equal in size. Each 

 radius has two median rows of rounded tubercular bases for 

 spines and two lateral double rows of ambulacral pores. An 

 examination of single plates shows that each ambulacral plate 

 has one tubercle and three pairs of ambulacral pores, a pair for 

 each foot. On the oral surface of the test the number of pores 

 increases gradually until near the peristome there are eight 

 pairs of pores for each radius as against two pairs of the aboral 

 surface. The equatorial adambulacral or interradial plates have 

 four tubercles each. This number decreases gradually towards 

 both poles until the last two aboral plates have only one tubercle 

 each, and the plates nearest the peristome only two tubercles 

 each. The radial plates bordering the peristome are in reality 

 the second oral radial plates. The first radial plates are inclosed 

 in the peristomial membrane. There are five pairs of them with 



