258 Mr. E. Billings on the Structure of 



the interposition of an exceedingly thin jjartitiou between the 

 circumambient water and the tiiiid within the general cavity 

 of the body. They are usually of a rhomboidal shape, each 

 rhomb being divided into two triangles by the suture (c c, 

 figs. 4, 5) between two of the plates. In several of the genera 

 the two halves of the hydrospires are reniform, ovate, or 

 lunate, and either internal or external. 



Fig. 4. Fig. 5. Fig. 6. 



A 



Fig. 7. 



» 



Fig. 4. Ilydrospire of Caryocrinus ornatus : a, surface view, the dots 

 around the margin are the spiracles, tlie small dotted lines represent 

 the course of the flat internal canals ; c c, suture between the t^^^o 

 plates ; 6, transverse section. Fig. 5. Ilydrospire of F/curoci/stifcs : 

 a, surface view ; c c, suture ; b, transverse section. Fig. G. The same, 

 with the points c c drawn together. Fig. 7. Internal gill of a spider. 



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

 them " hydrosjm-es,^^ and their apertures '^])ores,^^ '^fissures,^^ 

 or ^^ sjn'raclesj^^ according to their form. 



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

 rhomboidal form, and have each of the four 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 Avhen 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 hydro- 

 spire consists of a number of flat tubes arranged parallel to 

 each other and lying side by side, in the direction of tlie 

 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. 



