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. 



Eeference 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 oj)en the tentacular vestibule to the exterior and 

 expose the mouth in the centre of its floor. In Holopus, Hyocrinus, and Tliaumatocrinus 

 the orals persist as large triangular plates which cover up and protect the peristome 

 (PI. III. fig. 2 ; PI. 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 Rliizocrinus they are 

 relatively much smaller ; while they disappear altogether in the Pentacrinidse and 

 remaining Comatulse (except Tliaumatoa'inus), so that the mouth is directly exposed to 

 the exterior (PL LV.). 



In all the recent Crinoids the food-grooves of the disk are perfectly open, like those 

 of the arms, i.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 Palseocrinoids 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 Pal^ocrinoids. 



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 Pentacrinidse and Comatulse to a narrow tentacle furrow, recedes 

 in Palaeozoic 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 Palaeozoic 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. ^ Amer. Journ. Sci. 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. 



