478 MR. F. E. BEDDARD ON THE ATRIUM ASD [May 16, 



In my fuller paper upou the subject I did not insist upon this 

 comparison, which, however, I shall here again bring forward, 

 thougli for different reasons. 



Benham in his paper retains the name prostate for the glandular 

 appendices of Acanthodrilns, PericJueta, &c., allowing, however, 

 that the terminal part of these glands, which in PericJiceta 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 diiferent. 



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 diflerent famihes would lead to 

 the einmciation of somewhat impossible conclusions. Before com- 

 paring 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 me 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 some- 

 times quite large glands which are associated with the genital 

 papillae in the genus Pcricliceta. 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 bet\Aeen a single mass 

 of such gland-cells {Tuhifex) attached to a portion of the atrium 

 and a complete layer (Acanthodrilns) seems to justify this conclu- 

 sion. 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 diiferent families of the Oligochaeta 

 I shall commence with the higher groups. The tubular atria of 

 the Eudrilidse must be comparable to the somewhat similar atria of 

 the Acanthodrilidse &c. In Pontodrihis the sperm-ducts open into 

 the atria at the junction of the muscular and glandular parts ; in 

 the Eudrilidas 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 

 Acanihodrilus, Poniodrilus, ifec, are homologous with the compli- 

 cated lobate glands of PericlicBta, 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- 



