TL' Dr. W. B. Carpenter. [May 29, 



I. It has been noticed, as well by Lndwig as by Dr. P. H. Car- 

 penter, that in the terminal segments alike of the arms and of the 

 pinnules of Antedon Eschrictii, there is a similar want of the tentacular 

 apparatus, with an obliteration of the ambulacral grooves by the 

 approximation and fusion of the elevated folds of perisome at their 

 sides ; and that here also the sub-ambulacral nerve is absent. 



c. The most remarkable case of this kind, however, is presented by 

 that aberrant type of Comatulidae which is distinguished generically 

 as Actinometra. This genus differs from Antedon in the eccentricity 

 of its month : the anal orifice being generally in or near the centre of 

 its ventral disk, whilst the month lies near its margin. This curious 

 disposition is not related to any departure from radial symmetry 

 in the structure either of the calcareous skeleton, of its muscular 

 apparatus, or of the axial cords whose branches are distributed to 

 the muscles and perisomatic surface of the arms ; but it is associated 

 with a very marked irregularity in the disposition of the tentacular 

 and ambulacral apparatus. For, whilst in the arms given off from 

 the oral side of the disk, some pairs of pinnules are usually destitute 

 both of tentacles and of ciliated ambulacral grooves, this deficiency 

 is generally complete in a large proportion, not only of the pinnules, 

 but of the arms arising from the aboral side of the disk, sometimes 

 amounting to one-half of the entire circle, and occasionally also on 

 the disk itself. And wherever the tentacles and ambulacral groove 

 are wanting, the eub-ambulacral nerve also is absent. Yet we are 

 assured by Professor Semper, who kept Actinometrce for weeks 

 together in his aquaria, that " he never saw the least trace of any 

 irregularity in the alternating movement of their arms while swim- 

 ming ; " the non-tentaculated members, notwithstanding the want of 

 sub-ambulacral nerves, acting precisely like the tentaculated.* 



Thus, then, the " Experiment prepared for us by Nature," in 

 the subtraction of the sub-ambulacral nerve-system from certain 

 pinnules and arms of Comatulidre, entirely confirms the conclusions 

 which I drew from my artificial induction of the same physiological 

 condition. For it is, of course, impossible that their oral nerve-ring 

 can minister to the general sensori-motor actions of arms and pinnules 

 which receive no radial extensions of it; and yet the character of 

 those actions (as I have alrnady pointed out) so distinctly indicates 

 their dependence on the originating and co-ordinating power of a 

 nerve-centre, and on the internuncial power of nerve-cords, that we 

 must seek elsewhere for a nervous mechanism on which they depend. 

 Those who deny that this can be furnished by the axial cords, by the 

 dorsal centre from which they radiate (with the remarkable annular 

 commissure on its primary trunks), and by their minute ramifications 



* See Dr. P. Herbert Carpenter's Memoir on the genus Actinometra, in 

 " Linnn Transactions," New Series, TO!, ii, Zoology, p. 36. 



