706 DE. W. B. CARPENTER ON TIEE STRUCTURE, PHYSIOLOGY, AND 



cerned by Professor J. Mulleb, who was led by his comparison of the component pieces of 

 the calyx of Comatula Avith those of the calyx of Pentacrinus Caput-MeduscB to perceive 

 that the " centro-dorsal " or " knopf " of the former is the representative of the highest 

 joint of the stem of the latter ; the annulus formed by the adhesion of the Fust lladials 

 resting in each case upon its upper surface. The con-ectness of this view is placed 

 beyond all doubt by the study of the development of this " centro-dorsal plate :" for, as 

 I shall show in more detail hereafter (Sect. V.), this plate first presents itself in a form 

 which nowise differentiates it from the other joints of the cylindrical stem ; but begins 

 to take on an extraordinary increase in a peripheral direction at the time when the 

 dorsal cirrhi first sprout forth, and thenceforward remains in closer connexion with the 

 Calyx than with the rest of the Stem, from which it separates itself so soon as the dorsal 

 cirrhi are sufficiently developed to serve for the attachment of the animal. 



22. The Centro-dorsal plate' has the form of a shallow basin, with thick walls, and 

 lip turned inwards instead of outwards (Plate XXXIII. figs. 4, 5, G). Its outer mai-gin, 

 without departing much from the circular form, approaches the pentagonal sufficiently 

 to justify the designation given to it by J. S. Miller; and the margin of its inverted 

 jim, forming the boundary of the opening into its cavity, is also slightly pentagonaL 

 The diameter of this plate, in a full-grown Antedon, is about 'IG inch ; and its whole 

 depth '07 inch, of which about half is the depth of its cavity, and the other half the 

 thickness of its bottom. The thickness of the peripheral portion of its wall, how- 

 ever, to which alone the dorsal cirrhi are attached, is about twice that of the deepest 

 part of the basin; as is sho^vvn in the vertical section represented in Plate XXXV. 

 fig. 2. The central portign of the convex dorsal surface, by which the centro-dorsal 

 plate was originally articulated to the joint of the stem next beneath, is nearly flat, and 

 shows no peculiarity. But the entire peripheral portion is marked out into distinct 

 sockets for the articulation of the dorsal cirrhi. These sockets are more or less circular 

 depressions, separated by intervening ridges ; and from the bottom of each depression 

 there rises a tubercular elevation having a minute perforation in its centre. About 

 forty sockets may usually be counted in a full-grown specimen, disposed for the most 

 part in two rows, one alternating with the other (Plate XXXIII. fig. 5). Of these 

 sockets, however, there are usually some to which no cirrhi are attached ; these, which 

 are generally the nearest to the centre of the disk, are distinguished by the partial 

 filling-up of their cavity, so that the intervening ridges and the central tubercles become 

 less conspicuous, and by the absence of perforations in the latter. The meaning of this 

 difference will become obvious, when we follow out the development of the Centro-dorsal 

 plate and its appendages (§§ 75, 86-88), and mark the transference of the prehensile 



Miller's hjrpothesis was not so untenable as we h4ve since come to regard it ; but the unhesitating tone in 

 which the penetration of the stem by the Alimentary Canal is spoken of throughout Millek's Treatise, should 

 furnish a warning against any such assumption. 



' The " centro-dorsal piece " of many authors is compounded of the true eentro-dorscH plate and of the penta- 

 gonal base or circlet oijirst radials which closely adhere to it and to each other (§§ 23, 31). 



