1869.] BILLIN'QS — STRUCTURE OF CRINOIDS, ETC; 285 



In order to avoid the use of double terms, I propose to call 

 them ^' hydrospires,'^ and their apertures, " pores/^ ^'Jissures" 

 or "spiracles," according to their form. 



In Caryocrinus ornatus the hydrospires (fig. 4,) are of a 

 rhomboidal form, and have each of the lour sides bordered by a 

 single row of small tubercles. Some of these tubercles have a 

 single pore in the summit, while others are perforated with a 

 variable number, — from two to twenty, or perhaps more, — thus 

 becoming vesicular or spongy. It is only the apex of the tubercle, 

 however, that has this structure, for, when this is worn off, there 

 is only a single pore to be seen. The pores penetrate through 

 the plates, but do not communicate directly with the general 

 cavity of the body. Internally each hydrospire consists of a 

 number of flat tubes arranged parallel to each other and lying 

 side by side, in the direction of the dotted lines in fig. 4, a. 

 Each tube receives two of the pores seen on the exterior — one 

 pore at each end. These tubes are composed of a very thin 

 shelly membrane, which, although possessed of sufficient rigidity 

 to maintain its form, was no doubt of such a minutely porous 

 texture as to admit of the transfusion of fluids in both directions, 

 outward and inward. In a large hydrospire there are about 

 twenty of those tubes. Their greatest breadth is at their mid- 

 length, where they are crossed by the suture c c ; and as they 

 become narrower accordingly as their length decreases, the one in 

 the middle projects the deepest into the perivisceral cavity. In 

 consequence of this arrangement, when a section is made across 

 the hydrospire at the suture c c fig. 4 a, the form h is obtained 

 where c c is the surface of the shell, while the comb-like structure 

 below represents the tubes. 



Specimens of C. ornatus almost entirely empty are often found, 

 and in some of these the internal form of the hydrospires is 

 sometimes preserved. Those that I have seen have the form of 

 small rhomboidal pyramids, with four slightly convex sloping 

 faces, and composed of a number of vertical parallel plates — the 

 casts of the interior of the tubes — the substance of the tube itself 

 not being preserved. I have, however, several polished transverse 

 sections, in which I think the thin walls can be seen. 



The structure of the hydrospires is such, that there can scarcely 

 be any doubt that they are respiratory organs. The sea-water 

 entered through the pores, and aerated the chylaqueous fluid, 

 contained in the perivisceral cavity, by transfusion through the 



