THE ECHINIDEA. 487 



sphcera (Fig. 142, A) each ambulacral plate is thus divided 

 into three pore-plates, traversed 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 umbones, 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 with 

 each pair of pores, and therefore with each pedicel. Loven 1 

 has shown that the pore-plates are the primitive ambulacral 

 ossicles in the Ecliinoidea. 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 ambulacral 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 Astlienosoma, and in certain extinct 

 JEchinidea (Lepidoceiitrus, JEchinothurid), the plates of the 

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

 the extinct palaeozoic JPerischoechinidce, there are more than 

 two series of interambulacral plates, those in the middle of 

 each interambulacrum being hexagonal. 



In Echinus, the apical extremities of the ambulacra abut 

 upon the five smaller of the ten single plates which surround 



i " Etudes sur les Echino'id^es." (" Kongl. Svenska Vetensk-Akad. Hand- 

 lingar," Bd. ii., 1875.) 



