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Show 482 MR. F. E. BEDDARD ON THE ATRIUM AND [May 16, in one specimen of a species recently described by me (9) as Moniligaster bahamensis; in the other species investigated there was no trace of any perforation of the muscular layer by the outer layer of cells of the atrium : I imagine, however, that these prolongations of the cells were originally overlooked, owing to their inconspicuous character; they were especially obvious in the species Moniligaster bahamensis on account of the ducts being filled with granules of secretion, which were deeply stained by borax carmine. I compared this atrium with that of the Lumbriculidae and various other aquatic Oligochaeta, considering that the outer glandular layer of cells in both was the modified peritoneum. Benham, in the paper to which I have referred, agreed with me in this matter, but held that the cells, from the very fact that they were peritoneal, could not exert a glandular function; he doubted whether there was any known case of " coelomic epithelium being converted into a gland which pours its secretion to the exterior." This doubt appears to me to be quite justified, but then there is the undoubted fact that the processes of the cells in question do pour their secretion on to the exterior through the lumen of the atrium. The explanation of the difficulty seems to be that I was wrong in regarding this glandular epithelium as mesoblastic at all; I have never seen it myself, but Mr. Benham has shown me sections through the atrium of Moniligaster in which there was a distinct, though thin layer of peritoneum outside the glandular cells. The presence of this does not, however, invalidate m y comparisons, though it puts them in a different light, for Benham remarked apropos of Rhynchelmis that he could " detect in sections a delicate membrane surrounding the pear-shaped cells which is no doubt the true coelomic epithelium." The structure of the atrium in Moniligaster has also been studied by Rosa, who is disinclined to favour any " limicoline " affinities in that genus ; he principally uses the characters of a new Moniiigastrid, Besmogaster (14), to strengthen his position. In Besmogaster the atria have the following structure:-They are tubular glands slightly coiled in an S-shape; the lumen is much restricted, and is floored by a layer of cylindrical cells; outside this, the walls of the atrium are mainly muscular, the fibres being disposed centrally in a circular direction and longitudinally outside : among the external bundles of* fibres are groups of gland-cells which by means of long ducts (formed merely by a prolongation of their own substance) reach the lumen of the atrium ; but these groups of cells are not exposed at the surface of the atrium on account of the longitudinal muscular fibres; the entire organ is covered externally by a delicate layer of peritoneum. As Rosa points out, this seenis to indicate a gradual moving outwards of the pear-shaped cells from their primitive position as lining the lumen of the atrium. M y own criticism of this paper (as concerns the facts about the atria),'published three years ago, need not here be referred to, as the data upon which it rested are not sufficient. I am now quite of opinion that Rosa was perfectly right in looking upon the |
