Dr. F. E. Beddard on the Genus Trichodrilus. 237 



a layer of cells, around which are disposed muscular fibres 

 running in a circular direction; the outer layer is again 

 cellular. But the characters of these layers are totally 

 different — at any rate, of the outermost and innermost. The 

 middle muscular layer is simply much thinner. That there 

 is an outer layer at all is not obvious at first sight ; the pear- 

 shaped cells, closely pressed together, of the typical Tricho- 

 drilus (as shown in the figures of Claparede and Ditlevsen 

 and in my own sections) are replaced by a scanty layer of 

 cells, whose nuclei are visible, but at some distance from each 

 other. The cells are clearly flattened and few. So, too, 

 with the lining epithelium of the atrium. The general aspect 

 in fact of this organ is that of the " normal " atrium greatly 

 dilated, and its various layers therefore flattened through 

 pressure and extension. This may, of course, be the actual 

 fact ; but in the first place the difference of diameter may be 

 slightly, but is not greatly, in excess of that of the examples 

 reported upon above, and the sac is not gorged with sperm, 

 which might have been the cause of its dilation. The sperm 

 may, however, have escaped to the exterior or to another 

 individual. A nearly exactly similar difference in two indi- 

 viduals of Lumbriculus is figured by Mrazek *. 



Be this as it may, the condition of the sperm-ducts show 

 another kind of difference from those of the typical Tricho- 

 drilus icenorum. They were particularly easy to study on 

 account of their large size, which was not the case with 

 those of the other examples of the genus which I have 

 described in the present communication. 



The great increase of size was particularly marked in the 

 case of the anterior pair, in the middle of the course of which 

 the diameter of the duct was dilated to a width not very far 

 from that of the atrial cavity. A long piece of the sperm- 

 duct was thus increased. 



This increase of size of the sperm-ducts and of the cavity 

 of the atrium has brought it about that the entry of the 

 former into the latter is quite clear. They enter at opposite 

 sides — anterior and posterior — and at about the middle of the 

 atrium. It is not a question here of the dilation of the 

 sperm-ducts owing to pressure — at any rate, pressure which 

 has thinned and flattened out the walls. For the cellular 

 walls of the sperm-duct (surrounded, of course, by a flattened 

 peritoneal layer) are actually thicker than is the case with 

 the sperm-ducts of the individuals described above. I have 

 noted, indeed, that in some regions the whole sperm-duct of 



* Zool. Jahrb. xxiii. 1906, fig. F, p. 430, and fig. M, p. 440. 



