1893.] PB.OSTATE IJT THE OLIGOCH.f:TA, 



483 



atria of Besmogaster as offering an intermediate stage between 

 Monilkjaster and the higher Oligochteta. In the OHgochaeta we 

 haTe an atrium consisting of two layers of epithelium, covered 

 either by a simple thin peritoneum only, or (in the Eudrilidae) 

 consisting of thick muscular layers themselves covered with the 

 peritoneal epithelium. In a few simplified genera of Megasco- 

 licidse the lining of the atrium is reduced to the inner layer of 

 epithelium only : these are Ocnerodrilus, Gordiodrilus, Pi/gmceodri- 

 Im, and Ken-ia (an Acanthodrilid). It is, I imagine, fairly probable 

 that these forms are really, so far as concerns the atrium, degenerate 

 in character ; they show no real resemblance to the " Limicolae " 

 as I was at first inclined to believe ; for as I now show that the 

 same constituents are present in the atrium of the lower as of the 

 higher Oligochseta, the single layer of cells is a difference from both. 

 At one time the family Geoscolicidfe was defined by having no 

 atria like other families such as the Megascolicidse ; many genera, 

 however, do possess organs connected with the external pores of 

 the sperm-duets which seem to be referable to this category. 

 Eosa, however, in an account of the minute anatomy of these 

 organs in Kijnotm (15), is disposed to regard them as morpho- 

 logically different from the atria of the Megascolicida;. Unfor- 

 tunately the description of the structure of the organs in question 

 in Kynotus is unaccompanied by any illustrations, so that it is a 

 little difficult to follow. The terminal male apparatus consists of 

 a muscular bulb (" bursa propulsatoria ") attached to the walls of 

 the segment, on to which it opens by a retractor muscle ; from this 

 muscular sac arises a glandular tube of some length ; the muscular 

 sac is divided into two chambers by a transverse septum ; from 

 the dorsal one of these is given off the glandular appendix ; the 

 appendix is at first lined by a single layer of cells, and it is into 

 this part of it that the sperm-duct opens ; further back the lining 

 epithelium is composed of two layers, the outer one being a mass 

 of pyriform glandular cells. Eosa is of opinion that this structure 

 is not the homologue of the atria of the Megascolicidse ; its 

 resemblances, he thinks, are sui^erficial and, such as they are, are due 

 to convergence. The main reasons which Eosa gives for this opinion 

 are three :— (1) The fact that these " pseudoprostates " are found 

 only in a much specialized group of the Geoscolicidae ; (2) their 

 difference in structure from the true " prostates " of the Megasco- 

 licidse ; (3) the great resemblance which they show to the sacs 

 accompanying the copulatory setae of the same worms. The 

 second reason is further expanded into the actual points of 

 difference ; these are the following : — ^the presence of a retractor, 

 the complicated structure of the terminal sac, the mode of con- 

 nection of the sperm-ducts x^-ith the glandular appendix, and the 

 disposition of the glandular part. 



It appears to me that the differences are not so great as Eosa 

 would have us believe; it is true that if we compare, as he 

 suggests, Pontodrilus with Kynotus, the differences are more 

 striking than the resemblances. I, however, decline to regard the 



