202 Royal Society. 



they begin to undergo absorption. I can still trace tbeir basal 

 portions in young specimens of the free Antedon ; but as the 

 creature advances towards maturity they are altogether lost sight of. 

 When the intestinal canal has been sufficiently developed to open on 

 the surface of the oral disk, the anal plate is lifted out of the posi- 

 tion it originally occupied, and is at last found on the anal funnel, 

 far removed from the radials. This, like the oral plates, begins to 

 undergo absorption towards the end of the crinoidal stage, and 

 completely disappears in the early part of the life of the free Ante- 

 don. The radial plates increase not only in size but also in thick- 

 ness ; and channels which are left on their internal surface by 

 vacuities in the calcareous network, are converted into canals by a 

 further inward growth of this, which completely covers them in. 

 It is through these canals that the cords of sarcode pass to the arms. 

 The basal plates, like the oral, remain stationary in point of size, 

 and present no change in appearance or position until after they 

 have been completely concealed externally by the centro-dorsal piece 

 (the highest joint of the stem), which rapidly augments, both in 

 absolute and in proportional size, when the development of the dorsal 

 cirri is taking place from its convex surface. By the end of the 

 Pentacrinoid stage, this plate has extended itself so far over the base 

 of the calyx as completely to conceal the basals ; and as the free 

 Antedon advances towards maturity, it gradually extends itself over 

 the first radials, which then become adherent to it and to each other. 

 The basals then undergo a most curious metamorphosis, consisting 

 in absorption in one part and extension in another, by which they 

 finally become converted into five peculiarly shaped pieces, the 

 ultimate union of which forms the single rosettedike plate which 

 has been already stated to lie within the annulus formed by the first 

 radials of the adult Antedon. Hence the calyx finally comes to be 

 thus composed : — 



R 3 R 3 R 3 R 3 R 3 

 R 2 R 2 R 2 R 2 R 2 



b R 1 b R 1 b R 1 b R 1 b R' 



v 



Centro-dorsal. 



«- — yr- ' 



As the orals and the anal have entirely disappeared, no part of 

 the primordial calyx of the Pentacrinoid larva is traceable in it, until 

 we separate the adherent pieces which form its base, and search out 

 the minute and delicate rosette-like plate which is formed by the 

 metamorphosis of the basals. 



The structure, physiology, and development of the digestive, cir- 

 culatory, and respiratory apparatus, and of the nervous and muscular 

 systems, will form the subject of a future memoir. 



