STRUCTURE OP THE CYSTIDE^. 15 



point between the bases of the arms when these are present. 

 Through it the vessels of the aquiferous system and of the organs 

 of reproduction, which were situated in the grooves of the arms, 

 communicated with the interior. There can be little doubt also 

 that the nervous filaments, if the Cystideae possessed any, gained 

 access to the arms through this passage. 



Concerning the functions of these two apertures much difference 

 of opinion has existed amongst the best naturalists, some regarding 

 the large opening in the side as an ovarian orifice, others believing 

 it to be the mouth. Since the discovery recently made of the 

 ambulacral orifices of the true CrinoideEe and also of the arms of the 

 Cystideae, it appears to be quite certain that the latter opinion is 

 the correct one. I shall notice 'this subject more at length in the 

 next section. 



3. The anus. — In some species there is a third small aperture, 

 which is always situated near the apex. It is usually minute and in 

 certain genera has not yet been observed. This orifice is designated 

 the anus by most authors, a view of its characters somewhat sup- 

 ported by the fact that we know of no other function that can be 

 assigned to it. In the species which were not provided with this 

 openmg, the excrements were ejected through the mouth, as in 

 those Star-fishes that have no anus. 



IV. The Arms, Ambulacral Grooves and Pinnulce. 



The arms of the Cystideas only differ from those of the true 

 Crinoideee in the position which their bases occupy in the skeleton, 

 and in the general inferiority of their development. The structure 

 of the arms is essentially the same in the two groups, but in none 

 of the Cystideae do we find them of a very high degree of perfection. 

 I propose to arrange them in the following order : — 



1. Cystideae in which the body of the arm was not developed, but 

 only the grooves and pinnulae. The following species are examples: 



Cryptocrimis cerasus (von Buch), which in the specimens I have 

 observed, has fourteen small plates arranged in a circle around 

 the ambulacral orifice. Each one of these exhibits upon its surface 

 a small irregular scar, which marks the position of the attachment 

 of a single pinnula. No grooves are visible, but it is quite evident 

 that this species had no true arms. 



In Echino-encrinites angulosus (Pander), there were five or six 

 pinnulae, with their corresponding grooves. 



