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Show 478 MR. F. E. BEDDARD ON THE ATRIUM AND ' [May 16, In my fuller paper upon the subject I did not insist upon this comparison, which, however, I shall here again bring forward, though for different reasons. Benham in his paper retains the name prostate for the glandular appendices of Acanthodrilus, Perichwta, &c, allowing, however, that the terminal part of these glands, which in Perichceta receive the sperm-duct, may be compared with the atrium of the lower Oligochaeta. If this view is to be pushed to its logical conclusion, we are brought face to face with the somewhat puzzling conclusion that in Eudrilus half of the atrium is atrium and the rest prostate ; that is to say, two parts of a continuous tube which are perfectly identical in structure are morphologically different. A careful comparison of the various structures which have been called atrium by myself throughout the Oligochaeta seems to me to prove their identity beyond the possibility of a doubt; an attempt to distinguish between the atria of different families would lead to the enunciation of somewhat impossible conclusions. Before comparing in some detail the atria of different forms, a few preliminary points may be disposed of. I have urged, as already mentioned, the similarity in structure between the atrium and the clitellum ; this resemblance seems now to m e to be without the significance which I have attempted to attach to it. The atrium is, as far as we know, an involuted region of epidermis ; in many regions of the epidermis, not only in the clitellar region, there are unicellular gland-cells developed ; the most prominent of these perhaps are the sometimes quite large glands which are associated with the genital papillae in the genus Perichceta. I regard the glandular layer of the atrium as simply a thick layer of such unicellular glands ; and the fact that we find every possible stage between a single mass of such gland-cells (Tubifex) attached to a portion of the atrium and a complete layer (Acanthodrilus) seems to justify this conclusion. The resemblance to the clitellum is then simply due to the fact that in both sections of the epidermis there are glandular cells present in great quantity. In comparing the atria of the different families of the Oligochaeta I shall commence with the higher groups. The tubular atria of the Eudrilidae must be comparable to the somewhat similar atria of the Acanthodrilidae &c. In Pontodrilus the sperm-ducts open into the atria at the junction of the muscular and glandular parts ; in the Eudrilidae into the glandular part itself. As, however, there are so many variations as to the exact place at which the sperm-duct does communicate with the atrium, this can hardly be regarded as a difference of more than trifling importance. It will, I think, be generally allowed that the tubular glands of Acanthodrilus, Pontodrilus, &c, are homologous with the complicated lobato glands of Periclmta, Cryptodrilus &c. On a priori grounds only it seems impossible to distinguish glands which occur in such nearly related forms as Cryptodrilus and Megascolides. Perhaps if we had only to deal with Acanthodrilus and Perichceta the a priori grounds would not be so strong. Fortunately, how- |