542 
PROFESSOR W. THOMSON ON THE EMBRYOGENY OF ANTEDON 
With reference to the form and position of the oral plates, Professor Allman has sug- 
gested some interesting analogies between this transition stage of Antedon and the per- 
manent condition of the fossil genera Haplocrinus, Coccocrinus, Stephanocrinus , and 
Lageniocrinus. I thoroughly agree with Dr. Allman, that the oral plates of the Penta- 
crinoid are in all probability homologous with valve-like plates surrounding the mouth 
only in all crinoidal genera in which such plates occur. In Antedon rosaceus they dis- 
appear during the later stages in the growth of the Pentacrinoid young, and in all known 
species of the genus Antedon , even in those with a tessellated disk, they are wanting in 
the mature form. In Pentacrinus ( Neocrinus ) asterias , (L.), the mature form to which 
the fixed stage of Antedon is evidently most analogous, they are said to remain permanent. 
The evidence on this point is as yet extremely defective. It rests entirely upon the 
descriptions and sketches of M. Duchassaing *, which are sufficiently graphic, hut by no 
means technically exact. In two nearly allied species, Pentacrinus {Neocrinus) Mulleri 
(Oersted) and P. [N.) decorus (nob.), in both of which I have had an opportunity of 
examining the perisom of the disk, the oral plates are totally absent. 
Almost all Dr. Allman’s illustrations are necessarily taken from a small aberrant 
family of Crinoids, the Haplocrinidse, of whose structure we know as yet very little. With 
the exception of Stephanocrinus , which only doubtfully belongs to the group, all the 
genera are Devonian, preceded by the peculiar Cystideans of the Upper Silurians, and 
ushering in the carboniferous Blastoids. 
Notwithstanding Professor Mullek’s discovery of rudimentary free arms, I cannot 
help still leaning to the view that the triangular interradial valves in the Haplocrinidae 
may, like the pointed upper tier of interradial plates in the Pentremites, surround not 
only the mouth, but ovarian and anal openings ; a discussion of the homologies of the 
fossil Crinoids is however foreign to the object of the present memoir. 
The development of the assimilative and vascular systems, so far as it has been possible 
to observe it at this early stage, has already been described in detail. 
Explanation of the Plates. 
PLATE XXIII. 
Eig. 1. Portion of the ovary under slight pressure, showing ova in various stages of 
development, X 40 linear. 
Fig. 2, a-o. Ova in various stages, from the first appearance of the germinal spot 2, a 
to the maturity of the egg 2, o, X 40 linear. 
Eig. 3. Yelk-granules, X 120 linear. 
* Quoted by M. de Kokixck, “ Recherehes sur les Crinoi'des du terrain Carbonifere de Belgique,” p. 53. 
Brussels, 1854. 
