33 



taclied to the interior wall of the hucal ring. When the stomach is removed, the said sheath 

 appears (Tab. II, fig. 11, 12 c) like a short and thick pillar projecting convexly inwards and 

 e.xtending vertically along the interior wall of the bucal ring, connecting the bucal membrane 

 with the dorsal cuticle of the disc at the place where the madreporic body (a) is situ- 

 ated; it also partly incloses the upper end of the stone-canal (e). On cutting through the 

 exterior tendinous skin, the membranous 'organ in question (comp. Tab. VI, fig 6 h & fig. 8) 

 will immediately be discovered partly attached on the upper side by a ligament to the 

 stone-canal (s), and with its upper end attached to the dorsal skin at the same height as 

 the stone-canal, but somewhat inside or on the inner edge of the madreporic body. Its 

 other extremity extends considerably further downwards than the stone-canal, curving itself 

 round the thickened edge of the bucal ring, which below bounds the half- canal or duct 

 wherein the circular ambulacral vessel is situated. The organ is here attached by a broad 

 base to the skeleton of the disc. With regain! to the more particular structure of the organ, 

 it appears (fig. 8) like a membranous tube variously folded and contorted; its walls exhibit 

 under the microscope (see fig. 9) a fine net-like structure; thickened and thin places alter- 

 nating with each other. There may be plainly perceived in them light longitudinal fascicles 

 of fibres, partly anastomosing with each other and surrounded by a more opaque fine 

 granular tissue. In spirit specimens, the walls of the tube will usually be found collapsed 

 or in contact with each other; but in fresh specimens, this does not seem to be the case. 

 In the interior of it is found a fluid, which appears to be of the same nature as that con- 

 tained in the perivisceral cavity. The connexion o.f the upper part of the tube with the 

 dorsal skin might justify the assumption that it here terminates blindly. I have however 

 reason to suppose that there may yet be found here a sort of communication with the 

 perivisceral cavity; although I have not yet succeeded in indicating any such communication 

 with certainty. The lower end of the tube lies close to" the before mentioned circular 

 sinus, which is inclosed between the 2 lamellse of the bucal cuticle; and the tube stands 

 here, as I believe, in free communication with the sinus. 



Having as already mentioned, not been able to demonstrate any real ventral or 

 dorsal system of blood-vessels, nor to find in the Brisinga any special organs of respiration 

 which could make such a distinction of arteries and veins probable, I cannot attribute to 

 this apparatus the importance which has been ascribed to it. In any case it appears to me 

 that many circumstances forbid the assumption that we have here the real central organ 

 for the circulation of the blood. Of the probable distination of the organ I shall say some- 

 thing more in a subsequent section (Physiology). 



