DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) KOSACEUS. 743 



87. With this general augmentation in size, there is an increase both in the number of 

 sockets for the articulation of the Dorsal Cirrhi, and in the size of the individual sockets ; 

 and there is also a marked change in their disposition. I have not been able to satisfy 

 myself that after the development of the first tAvo whorls, each consisting of five cirrhi, 

 any similar regularity is observable in their subsequent multiplication ; but since, as I 

 shall more fully explain hereafter, the real origin of each cirrhus is in a peduncle of 

 sarcodic substance put forth from the central axis in the cavity of the centro-dorsal basin, 

 and since the arrangement of the whole aggregate of such peduncles is distinctly verti- 

 cillate, the want of a definite plan in the grouping of the cirrhi on the external surface 

 of that plate seems attributable to their very close apposition. The new cirrhi always 

 make their appearance between those previously formed and the base of the calyx, so 

 that their sockets are close to the margin of the basin ; and this is the position in which, 

 as already stated (§ 26), we find those rudimental cirrhi that often present themselves 

 even in the adult Antedon. The increase of the cirrhi in diameter is by no means 

 proportional to the increase in the diameter of the centro-dorsal plate ; so that not only is 

 space made on its surface for the augmentation in the number of their sockets from 10 

 to between 30 and 40, but a vacancy gradually comes to be left in the central part 

 of the exterior of the basin, which extends with its growth, and finally comes to bear a 

 considerable proportion to its diameter {^ 22). This vacancy cannot be accounted 

 for solely by the widening-out of the innermost circle of sockets by the general growth 

 of the basin in the manner already described ; and I feel no doubt that it is prin- 

 cipally due to a progressive exuviation of the first-formed cirrhi from within out- 

 wards, concurrently with the development of new ones near the margin; those cirrhi 

 which surrounded the summit of the stem being first shed, and their sockets being 

 filled up by new deposit; and the space thus formed being gradually widened by 

 the progressive exuviation of the cirrhi that bound it, and the filling -up of their sockets. 

 For in comparing a series of centro-dorsal plates in difierent stages of development, I 

 find distinct traces of such an obliteration of the original sockets ; and when we come 

 to study the connexions of the sarcodic axes of the cirrhi with that central Crinoidal axis, 

 of which the portion not left behind in the Stem is included within the Centro-dorsal 

 plate, we shall find additional evidence to the same effect. — Thus it appears that the 

 total number of dorsal cirrhi developed during the life of any individual Antedon, con- 

 siderably exceeds that which we meet with at any one epoch. How many of the earUer 

 cirrhi are exuviated before the latest ai-e put forth, I cannot state with certainty ; but I 

 should think that the number cannot average less than from 15 to 20, thus raising the 

 total to at least 50. 



88. Before leaving the Centro-dorsal basin, there is a further point of much interest 

 to be noticed in regard to its growth ; namely, the perforation of its wall for the passage 

 of the new twigs which are given off from the central axis from time to time, and 

 which, when they emerge on its external surface, become the sarcodic axes of the cirrhi. 

 It scarcely seems probable that such perforations should be left as vacuities in the 



MDCCCL.\VI. 6 1 



