156 THE VOYAGE OF H.M.S. CHALLENGER. 
ventral or oral side by a more or less solid integument, without external food-grooves or 
oral aperture ;” though they imply that the mouth may not‘have been internal in some 
cases. In recent Crinoids, however, the mouth and food-grooves are external, though 
capable of being closed by plates, and the name “ Stomatocrinoidea” was consequently 
suggested for them by Wachsmuth and Springer. 
Reference has already been made to the gradual removal of the orals which surround 
the tentacular vestibule of the larva, from the radial plates ; and also to the separation of 
these orals from one another so as to open the tentacular vestibule to the exterior and 
expose the mouth in the centre of its floor. In Holopus, Hyocrinus, and Thaumatocrinus 
the orals persist as large triangular plates which cover up and protect the peristome 
(PL. III. fig. 2; Pl VI. figs. 1-4; Pl. LVI. fig. 5). They are only removed to a short 
distance from the radials, scarcely at all in fact, in Holopus. In Rhizocrinus they are 
relatively much smaller; while they disappear altogether in the Pentacrinidee and 
remaining Comatul (except Thaumatocrinus), so that the mouth is directly exposed to 
the exterior (PI. LV.). 
Tn all the recent Crinoids the food-grooves of the disk are perfectly open, like those 
of the arms, 7.e., they are never closed in any other way than by the folding down over 
them of the plates at their sides (Pl. XVII. fig.6; Pl. XXVI. figs. 1, 2; Pl. XXX. fig. 2; 
Pl. XXXIX. fig. 2; Pl. LV.). But in many Paleocrinoids such as Actinocrinus, these 
food-grooves themselves were concealed beneath a vault or dome of rigid heavy plates ; 
so that the mouth towards which they converged was truly subtegminal. The nature of 
this dome is a point of very considerable importance with reference to the relationship of 
the Neocrinoids and Paleeocrinoids. 
Wachsmuth’ thinks that it “cannot in the remotest degree be homologised” with the 
ventral side of the Neocrinoids. “The solid dome forms, as I think I have proved, a 
continuation of the radial and interradial series of the dorsal side, and serves merely as a 
covering and protection for the organs underneath. It is in every sense of the word 
aboral, and forms a part of the abactinal system® [while the actinal system], which being 
already reduced in the Pentacrinidee and Comatulz to a narrow tentacle furrow, recedes 
in Paleozoic Crinoids one step farther and disappears within the solid walls of the body. 
The actinal system here consists externally only of the arm furrows, whence it continues 
underneath the vault. These Crinoids, therefore, are evidently of lower development and 
belong to an inferior type. ... The Paleozoic Crinoids, embracing therein all true 
Crinoids in which the actinal side is closed, represent the young stage of growth of the 
living types.” Elsewhere Wachsmuth* speaks of the ventral covering of Actinocrinus 
1 Revision, part i. p. 6. 2 Amer. Journ. Sct. and Arts, vol. xiv. p. 190. 
3 The words enclosed in brackets were unfortunately omitted in the original, thereby confusing the author's 
meaning not a little. 
* Revision, part ii. p. 14. 
