DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) E0SACEU8. 74X 



tacriniis 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 Anfedon. 



82. The only portions of the skeleton of the young Antedon now remaming 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 five Orals have undergone consists in a further 

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

 noticed {^ 08). This process goes on with considerable rapidity during the period at 

 which the pedunculate Fentacrinoid 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 diflferent speci- 

 mens. In some we 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 

 (§ 64) ; 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. 



8.3. 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 Kokinck 

 and Le Hon {op. cit.), as consisting of the following pieces : — 



Basals 5 



Eadials 5x3 



T ,. , rfirst, 1 (Anal). 



Interradials ■{ , V /^ i ^ 



Lsecond, 5 (Orals). 



Brachials 10. 



84. Professor Wrv'iLLE Tiiomsox, however, regards the skeleton as composed of two 

 systems of plates, the radial, and the perisomatic ; which he states to be "thoroughly 

 distinct in their structure 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 pumulcs. The Perisomatic system includes the basal and oral plates, the 

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

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

 arms (§§ 39, 81). Whilst partly agreeing Avith 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 jouits or plates of the radial system," he says, " may be at once 

 distinguished by their being chiefly made up of the peculiar fasciculated (or radial) tissue 



' Philosophical Transactions, 1865, p. 540. 



