73G ON THE YOUNG STAGES OF ECHINI. 



found in the adult) are next formed between them and the teeth (Echinus- 

 like), while afterwards they cover the whole membrane, as in Toxopneustes 

 (PL VII. f. .'"i- and some of the species of Echinus and Trigonocidaris 

 (PI IV. f. i), appearing between the ten plates and the test ( /'/. VII. f. 

  ■>. U. !',)■ This mode of growth is totally unlike the growth of the movable 

 imbricating buccal plates of the Cidaridae ( /'/. II. f. 9, //). where these plates 

 perform the part of ambulacra] and interambulacral plates, and appear near 

 the test at first, forming in full-grown specimens rows made up of more than 

 two plates, as in the Palaechinidae, suggesting that the test of Palaechinidae 

 must have been made up of plates homologous to the buccal plates of Cidaris. 

 The test, of course, would then have been capable of considerable com- 

 pression and change of outline, as is the case in Astropyga and Asthenosoma. 

 This similarity is very striking in young Cidaridae, where the number of coro- 

 nal plates is small, and the young sea-urchin seems to consist almost entirely 

 of an abactinal and an actinal system, separated by a narrow band of 

 coronal plates. 



In some species of Temnopleuridae the subanal plate remains very prom- 

 immt.even in adult specimen- | /'/. VII. f. ?$) ; the anal system in the voting 

 is covered by one large elliptical plate; as the anal system enlarges, numer- 

 ous minute plates surround the larger plate (/'/. /'//./'. .-;'), which always 

 retains its peculiar ornamentation, and is readily distinguished from the 



others by its size and shape. In others, on the contrary, the anal system 

 undergoes changes identical with those of Strongylocentrotus, Echinus, and 

 the like. In Temnopleiirus the pits ;it the angles of the plates appear al first 

 like rectangular openings ( /'/. VIII. f. .-;). which, as the specimens grow 

 older, become little by little connected by grooves, -rowing deeper and more 

 prominent with advancing age ( /'/. VIII. f. .v. ;:,). In another species of 

 the genus the pits are never so marked in the adult, becoming simply comma- 

 shaped ( /'/. VII. f. 26). The miliaries of the Temnopleuridae are formed, 

 as in other genera, by ridges appearing at fust connected with the base 

 of the primary tubercles ( VI. VIII. /'. .;. //, / ;. a. .>;). In Trigonocidaris the 

 young [PI. IV. f. 7) differ from the old (PI IV.f..:) in having larger 

 pits, less numerous and lower ridges, and Imt \\'\v secondary tubercles, the 

 principal rows of ambulacral and interambulacral tubercles being very promi- 

 nent. The buccal membrane and the abactinal system present no striking 

 differences; there were only four anal plates in all the specimens collected. 

 In Temnechinus, of which an extensive series was collected, we find in the 



