ECHINODERMATA. ECHINOIDEA. 75 



one has a porous structure serving as the madreporic 

 plate (ra). Outside the genital plates and alternating 

 with them come the ocular plates, these are smaller than 

 the genital and usually triangular or pentagonal and they 

 each have a perforation for the eye- spot. In the irregular 

 echinoids, the anus is not placed within the apical disc, 

 which then becomes more compact. In some forms it is 

 considerably elongated, as in Collyrites, in which two of 

 the ocular plates are placed some distance posterior to the 

 other portion of the disc. 



The corona consists of twenty rows of plates, each row 

 extending from the apical disc to the mouth. The plates 

 are of two kinds, ambulacral (fig. 19 A, a) and interam- 

 bulacral (b) and they are arranged in pairs, there being 

 five double rows of ambulacrals separated by five double 

 rows of interambulacrals, each double row being termed 

 an area. The former end against the ocular plates, the 

 latter against the genital, and in each case fresh plates 

 are developed next the apical disc. In each area the 

 plates alternate on either side, and since their inner ends 

 are triangular, the line between the two rows is zig-zag. 

 The ambulacral plates are smaller and more numerous 

 than the interambulacral, and they are perforated by 

 pores (p) for the passage of the tube feet to the exterior, 

 a radial water-vessel being placed under each ambulacral 

 area. The pores may be round or elongated and they are 

 situated in the outer portion of the plates and are almost 

 always in pairs, sometimes there is one pair on each plate, 

 sometimes more. When there is one pair, the pores are 

 said to be unigeminal, two pairs bigeminal, three pairs 

 trigeminal. In the two latter cases the pairs are arranged 

 obliquely. Sometimes the pores in each pair are united 

 by a groove, they are then termed conjugate. In some 



