

THE mil MIH-: \ 487 



sphwra (Fig. 142, A) each ambulacra! plate is thus divided 

 into three pore-plates, ii.i\< i>d Altogether by six pores, or 

 short canals. The outer openings of these canals are arranged 

 close together in pairs upon little, excavated, shield-shaped 

 elevations, or umbo?ies, sculptured on the outer or interam- 

 bulacral half of the face of the ambulacral plate ; but their 

 inner extremities are much wider apart. A pore-plate, or 

 subdivision of the ambulacral plate, thus corresponds \\ith 

 each pair of pores, and therefore with each pedicel. Loven * 

 has shown that the pore-plates are the primitive ambulacra! 

 ossicles in the Echinoidea. At its apical extremity, in fact, 

 the ambulacrum is composed of only two small ossicles, which 

 meet in the middle line. Each of these primitive ambulacral 

 ossicles is perforated by a single or double pore for the pedi- 

 cel which it bears. But as, in the course of the growth of the 

 corona, new primitive ainbulacral ossicles are added between 

 the ocular plate and those already formed, the latter shift 

 toward the oral end of the ambulacrum, and grow in corre- 

 spondence with the larger space which they have to fill. But 

 they grow unequally ; and while all retain their primitive con- 

 nections with the adjacent interambulacral plates, some lose, 

 while others retain, their median union with the correspond- 

 ing ossicles of the same ambulacrum. The former, therefore, 

 are, as it were, pushed away from the middle line by the union 

 of their encroaching predecessors and successors. Groups of 

 the primitive ambulacral plates, thus modified, enter into close 

 union, and constitute the complex ambulacral plates of the 

 fully-developed ambulacrum. 



In the genus Cidaris, the primitive ambulacral plates en- 

 large, but do no coalesce into secondary ambulacral plates ; 

 hence the distinction between ambulacral plates and pore- 

 plates vanishes. The ambulacral plates are continued on the 

 peristome to the margins of the mouth, and here they become 

 somewhat altered in form, and their edges overlap. 



In the living genus Asthenosoma, and in certain extinct 

 l\cli'mnlea (Lepidocentrus, Echinothurid], the plates of the 

 corona are loosely united anU overlap one another ; while, in 

 the extinct palaeozoic Perischoechinidce, there are more than 

 two series of interambulacral plates, those in the middle of 

 each intoranilmlaerum being hexagonal. 



In Et'hhnix^ the apical extremities of the ambulacra abut 

 upon the Hve smaller of the ten single plates which surround 



" Etudos sur les Echinold4es." (" Kongl. Svenska Vetensk-Akad. Hand- 

 ,'' Bd. ii., 1875.) 



