TYPE ECHINODERMA. 571 



this tentacle, these plates are usually knowii in this group 

 as the oculars. Starting from each ocular and each genital, 

 rows of plates pass outwards and downwards towards the 

 mouth, the test being formed of twenty such rows extending 

 in meridional lines from the aboral to the vicinity of the oral 

 pole. The rows are grouped together in pairs, five of the pairs 

 starting from the genital plates and the other five from the 

 oculars ; and since a number of the plates in these latter rows 

 are perforated for the emission of tube-feet, they are generally 

 known as the ambulacral plates (Fig. 262, A], while those of 



m 



---' A 



I 



FIG. 262. FIGURE SHOWING THE ARRANGEMENT OF THE APICAL SYSTEM OF 



PLATES OF Strongylocentrotus. 



A = ambulacral areas. / = interambulacral areas. 



an = anus. m = madreporite. 



g = genital plates. o = ocular plates. 



the intervening pairs are termed the interambulacrals (/). The 

 pores for the tube-feet are almost always double, and are 

 situated usually near that side of a plate which abuts upon 

 the adjacent interambulacral ; in some forms but a single 

 pair of pores occurs on each plate, but more usually two or 

 more pairs are found (Fig. 262) an arrangement which indi- 

 cates that such plates are formed by a fusion of several 

 smaller ones, each of which is represented by one of the pairs 

 of pores. 



In the more primitive forms one of the genital plates is 

 transformed into the madreporiform tubercle (Fig. 262, m), 

 but in others the limits of the tubercle may extend so as to 



