DETELOPMEXT OF AXTEDOX (COMATIJLA, LAMK.) KOSACEUS. 747 



bifurcates, is proi^rcssively enlarged by the internal absorption of its wall, as in the pre- 

 ceding cases. 



94. Oral Plates. — The removal of these first-formed plates by absorption, which we 

 have seen to commence before the termination of Pentacrinoid life, is completed vciy 

 soon after the young Antedon has entered upon its free stage of existence. The absorp- 

 tive process continues to take place from above doAvnwards ; and the last traces of those; 

 plates that can be distinguished, are seen as glistening fragments of calcareous network 

 at the bases of five membranous valves which still fold over the tentacles forming the 

 oral ring, in specimens which have attained a diameter of about an inch and a lialf. 

 These soon disappear entirely. 



95. Anal Plate. — This plate is still distinguishable in specimens that show no vestiges 

 of the Orals ; but it has undergone no increase in superficial dimensions, and is so far 

 from being augmented in thickness that it seems rather to have been thinned by inci- 

 pient absorption over its whole surface, preparatory to its complete disappearance a short 

 time after. I do not find that either the upper part of this plate disappears before the 

 lower (as we have seen to be the case with the Orals), or the lower before the upper ; 

 and as I have found no vestiges of it, though I have carefully sought for them, in young 

 Antedons of about 2 inches in diameter, I conclude that the entire plate is removed at 

 once by a continuance of absorption over its whole surface. 



96. Arms. — It does not seem requisite to follow in any detail the development of the 

 segments of the Ai-ms; since the changes in conformation through which the first 

 formed Brachial segments pass in their progress to maturity, are precisely analogous to 

 those which have been already described in the Eadials ; and the successive production 

 of new segments at the extremities of the arms takes place after exactly the same fashion 

 as in the Pentacrinoid stage. The general tendency is to an increase in the diameter of 

 the segments, which is relatively much greater than the increase in their radial length 

 (§ 40) ; and with this there is an extension of their apposed articular faces, which gra- 

 dually come to present the ridges and fossa) characteristic of the adult type, whilst at 

 the syzygies these faces are brought into closer mutual approximation. The central 

 canal by which each segment is traversed, undergoes a corresponding enlargement. The 

 development of the Pinnee from the basal portions of the arms, which for the most part 

 remain destitute of them for some time after the first appearance of these lateral ap- 

 pendages (§ 69), takes place by the time that the Orals disappear; but these interme- 

 diate pinnae are long in attaining the dimensions of those nearer the terminations of the 

 arms. The increase of the Pinna? in length, like that of the Arms themselves, partly 

 depends upon an increase in the number of their segments, and partly on an augmen- 

 tation in the length of each individual segment. How ^and where the new segments 

 are added, I cannot certainly say ; but it may be safely assumed that they are not deve- 

 loped at the terminations of the pinnules, since their peculiar terminal hook is formed 

 when as yet the segments are few in number. And as the basal segment has a peculiar 

 conformation for its articulation witli tlio brachial from which the pinnule proceeds 



