194 P. HERBERT CARPENTER. 



He regards them as the " rudiments of the first primary 

 plates of the ambulacra," which are arrested in their 

 extension towards the peristome by the resistance of the 

 auricles that are attached to the internal surfaces of the 

 ambulacral plates, and serve as supports to the lantern. 

 For between every two pairs of these, nearer the periphery, 

 he finds smaller triangular plates intercalated, which he 

 regards as the first traces of the interradial areas. In the 

 Cidaricla, however, the bases of the auricles are interradial, 

 and they therefore offer no resistance to an extension of the 

 ambulacra towards the margin of the corona. As the plates 

 successively reach this margin, their sutures are opened and 

 they undergo considerable changes, so as to give rise to the 

 imbricated plates of the actinal membrane, which are 

 therefore merely metamorphosed ambulacral plates. In 

 many Urchins {EcJmitis, Temnechinus, Strongylocentrotus) 

 the actinal membrane is quite bare, with the exception of 

 the ten perforated buccal shields. These are formed very 

 early near the edge of the test, but gradually approach the 

 bases of the teeth during development. In certain genera 

 [Porocidaris) small imbricating plates are formed between 

 them and the teeth, Avhile the remaining peripheral part of 

 the membrane is left bare. More commonly, however, this 

 is covered by imbricating plates in greater or smaller number. 

 Thus, in Hemtpedina the ten buccal plates are large and 

 occupy nearly the whole membrane, which bears eight or 

 ten very much smaller ones between them and the test. 

 But in Salenia and Toxopneustes the membrane is chiefly 

 covered by a number of imbricating plates closely packed 

 together, though the ten perforated buccal plates remain 

 distinct. In Trujonocidaris they are but slightly more 

 prominent than the others, and in the Diadematidce and 

 Cidaridce all trace of them is lost, at any rate in the adult, 

 as seems also to be the case among the irregular Urchins. 

 In EcMnothrix and in the EcJmiothurida the actinal 

 membrane is covered, as in the Cidaridce, by a number of 

 movable imbricating plates, which perform the part of 

 ambulacral and interambulacral plates, like those of the test 

 of an ordinary Urchin. For the imbricated plates continuing 

 the ambulacral system on to the actinal membrane are 

 pierced for suckers identical with those of the rest of the 

 ambulacral zone, extending unbroken to the actinostome as 

 far as the buccal tentacles, which are not different in size 

 from the other ambulacral tentacles. 



These imbricating buccal plates which Loven regards 

 merely as metamorphosed ambulacral plates, form a much 



