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Show 1893.] MR. A. E. SHIPLEY ON THE GENUS SIPUNCULUS. 331 canal, but into a well-developed system of lacunar spaces which lies in the thick walls of the rectum. Section shows that the whole organ is hollow; the walls of the finer branches are one cell thick, and these cells seem to be shedding their nuclei into the surrounding medium. The lumen of the branches, and more especially of the main trunk, contains a granular coagulum in which numerous spherical granular corpuscles are embedded; these latter have very much the appearance of the nuclei described above as being given off from the cells of the branches, and it is quite possible that the latter are not all nuclei, but some of them may be bodies elaborated in the lumen of the organ and passing through the walls to the exterior, that is, into the coelomic fluid. The lacunar spaces into which these organs open are well defined (Plate X X V I I . fig. 10); they can be recognized without the aid of sections, for if the rectum be cut out and examined under a lens it becomes apparent that it consists of a thick inner tube surrounded by a thinner, looser tube, which is supported by four longitudinal mesenteries attached to the body-wall (Plate XXVI. fig. 9). The space between tbe outer and inner tubes is the space into which the rectal diverticula open, and its cavity contains a coagulum similar to, and continuous with, that in the cavity of the branching organ. The lacunar spaces do not extend any great distance along the intestine, but are confined to a short track about | an inch long; they are split up by numerous strauds of connective tissue which run between the outer and inner walls of the rectum. Until I came to investigate minutely the structure of the anal tufts I had always regarded them as homologous with the anal caeca of Bonellia, &c. Both their appearance and position seemed to support this view. Closer study, however, shows that the structures in Sipunculus differ very considerably from those in Bonellia. In the first place, they do not open into the ccelom : the ciliated funnels at the end of the branches in the anal caeca of armed Gephyrea are well known, nothing of the sort is found in Sipunculus. Secondly, they do not open into the lumen of the intestine but into a well-developed system of lacunar spaces in the wall of the rectum. These differences seem to throw much doubt on the view that any homology exists between these structures in the two groups. If we may make any inference from the structure of the gland to its function, it appears probable that this branching gland has somewhat the same functions as the lymphatics and the numerous glands which in all classes of animals exercise some influence on the constituents of the circulating medium. List of Papers referred to. (1) KEEERSTEIN, W . - " Beitrage zur anatomischeu und system-atischen Kenntniss der Sipunculiden." Zeitschr. f. wiss. Zool. Bd. xv. |