DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 741 



tacnmis Caput-Medusce. It is curious that this perisomatic skeleton of the Arms, like 

 the Oral plates {^^ 82, 94), subsequently undergoes complete absorption, so that no 

 trace of it is discoverable in the adult Antcdon. 



82. The only portions of the skeleton of the yoixng Antedon now remaining to be 

 described are the Oral and the Anal plates ; and respecting these there is but little to 

 be said. The only change which the live Orals have undergone consists in a further 

 advance of the process of absorption, the commencement of which has been already 

 noticed (§ 68). This jn-ocess goes on with considerable rapidity during the period at 

 which the pedunculate Pentacrinoid is being transmuted into the free Antedon ; and as 

 the epoch of its detachment from the stem is not always precisely the same [^ 72), so 

 the amount that has been removed by absorption at that epoch varies in different speci- 

 mens. In some Ave find the upper half or even two-thirds of each oral plate to have 

 entirely disappeared ; whilst in others the marginal portions only of the upper part of 

 the plate have been removed, leaving a sort of central tongue projecting upwards from 

 the basal portion. The single Anal plate (Plate XLI. fig. 2, a) still retains the elliptical 

 form which characterized it from the time when it was lifted out from the circlet of radials 

 [^ G4) ; and it seems to have undergone but little change in any of its dimensions, either 

 by addition or absorption. It is still a simple cribriform film, of which the lower portion 

 shows a closer texture and a more uniform margin than the upper ; and it is not con- 

 nected with any other portion of the skeleton, save by the general perisomatic substance. 



83. The entire skeleton of the Calyx — putting aside the centro-dorsal piece as really 

 belonging to the stem — may be described, according to the formula of MM. de Koni>'CK 

 and Le Hon [op. cif.), as consistmg of the followmg pieces : — 



Basals 5 



Eadials 5x3 



^. , ffirst, 1 (Anal). 

 Intenadials \ , r /^ i > 



Lsecond, 5 (Orals). 



Brachials 10. 



84. Professor "Wya'ILLE Thomsox, however, regards the skeleton as composed of two 

 systems of plates, the radial, and the perisonuitic ; which he states to be " thorouglily 

 distinct in their structui-e and mode of growth"'. The Radial system consists of the 

 joints of the stem, the centro-dorsal plate, the radial plates, and the segments of the 

 arms and pinnules. The Perisomatic system includes the basal and oral plates, the 

 anal plate, the intcrradial plates sometimes seen between the second radials (§ 39), 

 and any other plates or spicules that may be developed in the perisome of the disk or 

 ai-ms (§§ 39, 81). Whilst partly agreeing with him on this point, I find myself unable 

 to accept his distinction to its full extent, since its basis is not in harmony with my own 

 observations. . " The joints or plates of the radial sjstem," he says, " may be at once 

 distmguished by their bemg chiefly made up of the peculiar fasciculated (or radial) tissue 



' Philosopliical Transactions, 1805, p. 540. 



i 



