REPORT ON THE CRINOIDEA. 411 
gradually come to lie in canals within the radials, and so are relatively (though 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 Gétte’s observations, the ciliated ambulacral epithelium of the Crinoids is 
' so that the ambulacral nerve, 
derived from the cellular lining of the left peritoneal sac ; 
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 Gétte’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 Paleeocrinoids 
which had imperforate radials, just as they are in the Pentacrinoid larva. A variation of 
the same nature, though of course altogether different 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 
2 
seem to quote me as having noted its occurrence in Actinometra. Ludwig* long ago 
1 Archiv. f. mikrosk. Anat., Bd. xii. pp. 591-593. 
2 Beitriage zur Anatomie der Crinoideen, Nachricht. v. d. kgl. Gesellsch. d. Wiss. zw Gottingen, No. 5, 1876, p. 108. 
