DEVELOPMENT OF AKTEDON (COMATULA, LAMK.) liOSACEUS. 711 



J. MuLLEU, ^vllO first noticed its presence, to be occupied by a nutrient wssi*/, proceeding 

 from an organ contained in the basin-shaped cavity of the Centro-dorsal plate, uhich he 

 designated as a hcaH, and from which he asserted that similar but much larger vessels 

 pass off into the Radial plates, and thence by bifurcation into the Arms. I am perfectly 

 satisfied, however, that these axial canals are occupied by cords of unconsolidated sarcodic 

 substance ; and tliat the central organ from which they proceed is developed out of the 

 orio-iual Crinoidal Axis. How far these cords are subservient to the nutrition of the 

 (u-gan, or to the maintenance of its vitality, is a question that will be fully considered 

 when the structure and function of the central organ to which they are related come 

 under review, in the Second Part of this Memoir. 



30. Having thus described the structure of the typical Cirrhus and of its component 

 pieces, I have to speak of the cirrhi whose condition departs more or less Avidely from 

 that type. In almost every specimen of Antedon we find cirrhi which do not present all 

 the characters of maturity ; and there are very commonly some whose condition is quite 

 rudimental, corresponding to that which will be described in detail when the develop- 

 ment of the several pieces of the skeleton is being traced out (§ 06). Such a one, repre- 

 sented at b (Plate XXXII. fig. 5), is seen to consist of eight minute cylindrical segments, 

 of which the basal is the largest, and of which the terminal is rounded ofi" without any 

 appearance of a claw. At c is shown a more advanced stage of the same rudimental 

 form; the segments having increased in length and diameter, but without showing any 

 other change. At d we have a cirrhus which still presents the same rudimental form of 

 the segments, but these have increased in number to ten, and the last segment carries 

 a small claw ; and the same condition is still presented at e, though the number of seg- 

 ments has increased to twelve. At /, however, we not only see an increase in the size 

 and number of the segments, of which there are sixteen besides the terminal claw, but 

 there is an incipient bevelling-off" of the opposed faces of the segments on theii- aboral 

 side, which indicates an advance in development towards the mature type ; and the 

 basal segments are equalled iu diameter by those which follow. It is comparatively 

 rare, however, to find the rudimental form still exhibited by cirrhi which have attained 

 dimensions so considerable ; the shaping-out of the segments often taking place when 

 they are still of very small size, and the terminal claw presenting its characteristic form 

 almost from the first. This course of development is seen in the series marked 

 g, h, i, /.•, t, m ; — in which it is to be observed that the basal segments are even from 

 the first of no larger diameter than the rest ; that the mature proportions between the 

 length and breadth of the segments are shown at a very early period (Ii) ; that tlie bevel- 

 ling-off of the opposed faces on the aboral side takes place (?) when both in number and 

 dimensions the segments are very immature ; and that in a cirrhus (/) whose length and 

 diameter do not exceed one-eighth of the normal standard shown at a, all the characters of 

 maturity are presented by the individual segments, even to the development of the oppo- 

 sing process on the penultimate segment. Hence between this and the typical mature 

 cirrhus, the only difterence consists in the numher and size of the segments. At n is seen 



MDCCCLXVI. 5 E 



