97 



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. 53, 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. 53 a, the 

 form 6, is obtained where c, c, is the surface of the shell, while the comb- 

 like structure below represents the tubes. 



Fig. 53. Fig 54. Fig. 55. 



Fig. 53 Hydrospire of Cari/ocrinus ornatus. surface view, the dots around th e margiQ 

 are the spiracles, the small dotted lines represent the course of the flat internal canals 

 c, c, suture between the two plates; 6, transverse section. Fig. 54. Hydrospire of 

 Fleurocystites. surface view ; c, c, suture ; h, tranverse section. Fig. 55 (/. The same 

 with the points c, i', drawn together ; 6, internal gill of a spider. 



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 

 tranverse 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 

 throughthe pores, and Derated the chylaqueous fluid, col» ♦"lined in the 



