KEPORT ON THE CRINOIDEA. 411 



gradually come to lie iu canals withiu the radials, and so are relativel}^ (tliougli of course 

 not absolutely) nearer the epidermis on the exterior of the body, which in this case is 

 antambulacral. 



The ontogenetic change in the relative position of the axial cords of a Crinoid is thus 

 directly the opposite of that which Marshall supposes to have taken place phylo- 

 genetically. On his theory the antambulacral portion of the primitive nerve-sheath 

 should commence by being outside the radials, between them and the epidermis ; 

 whereas, as he himself admits, the radials are at first in the form of " calcareous plates 

 between the cords and the integument." His argument is, therefore, only one of analogy, 

 and the outward movement of the axial cords in the Crinoid larva is not comparable 

 morphologically to the inward movement, which must have taken place during the develop- 

 ment of the radial nerve of an Urchin from the primitive nerve-sheath of a Starfish. 



It appears to me, however, that there is a possible view of the phylogeny of the 

 axial cords which would not conflict in this way with their ontogenetic movement. 

 According to Gotte's observations, the ciliated ambulacral epithelium of the Criuoids is 

 derived from the cellular lining of the left peritoneal sac ; ' so that the ambulacral nerve, 

 which is in such close relation with this epithelium, is endodermic in its origin. On the 

 other hand, the axial cords of a larval Crinoid lie in the walls of the dorsal portion of the 

 body-cavity, which is lined by an epithelium derived from that of the right peritoneal 

 sac ; and I would therefore suggest that the embryonic axial cords might have the same 

 primitive relation to this epithelium as the ambulacral nerves must have to that of the 

 left peritoneal sac, if Gotte's observations be' correct. The outward movement of the 

 cords during development, away from the epithelium of the body-cavity, would then be, 

 so to speak, serially homologous with the supposed inward movement of the ambulacral 

 nerves of an Urchin. In other words, the axial cords are the nerves of the right 

 antimer, while the subepithelial bands of the ambulacra are those of the left antimer, 

 both being derived in the first instance from the epithelium of the enterocoel. 



In the Neocrinoids the axial cords eventually come to be some little distance from 

 this epithelium ; though they must have remained close to it in those Palseocrinoids 

 W'hich had imperforate radials, just as they are in the Pentacrinoid larva. A variation of 

 the same nature, though of course altogether difi"erent in degree, presents itself in the 

 relation of the ambulacral nerve in certain species of Neocrinoids. I refer to the presence 

 or absence of a delicate connective tissue lamella between the nerve and the overlying 

 epithelium. This layer is often very conspicuous in Antedon eschrichti, but occasionally 

 seems to be absent ; while its presence in Antedon rosacea is doubtful. I have never 

 satisfied myself of its existence in any other species, though Ludwig and Marshall 

 seem to quote me as having noted its occurrence in Actinometra. Ludwig^ long ago 



1 Archiv. f. mikrosk. Anat, Bd. xii. pp. .591-.'59.3. 



2 Beitriige zur Anatomie der Crinoideeii, Nachrichf. v. (I. kijl. Gesdlsch. d. {Viss. zu Gottinfjen, No. 5, 1876, p. 108. 



