116 OLIGOCHAETA 



incline to the latter view, and will here state my reasons for so doing. The fii'st 

 reason which leads me to this opinion is entirely a priori ; this is, however, in the 

 present case a more powerful reason than it might be supposed to be, and than it 

 would be perhaps in some other cases. The Oligochaeta so evidently form a single, 

 well-definable group that the structures of the terrestrial forms may be confidently 

 expected to be represented in the aquatic genera; at any rate we know of no 

 structures which are peculiar to one or the other except those now in question. 



No one, I imagine, will doubt that all the structures called ' atrium ' in the 

 aquatic genera are homologous with one another. With inconsiderable exceptions 

 these 'atria' receive the sperm-ducts; they have even been (erroneously) spoken of 

 as an ' enlarged part of the sperm-duct.' In the Eudrilidae and in those Geoscolicidae, 

 where they exist, the glandular tubes at the male pore also receive the sperm-ducts ; 

 in Eudrilua itself, for example^ the sperm-ducts open into the middle of the 

 glandular tube ; it can hardly be doubted that in this case we are dealing with 

 a structure that does accurately correspond to the 'atrium' of the lower Oligochaeta. 

 In the Acanthodrilidae, for instance, another arrangement occurs : here the structures 

 which I have termed spermidueai glands open quite independently of the sperm-ducts, 

 even on to a difierent segment; and yet the histology of the gland is in its main 

 features quite like that of the Eudrilidae. The link between the two is aflforded by 

 a series of genera ; in Pontodrilus the gland is not independent of the sperm-duct ; 

 but the sperm-duct opens into the muscular duct and not, as in the Eudrilidae, 

 into the glandular region ; in Typhaeus and in Microscolex the sperm-duct only 

 communicates with the gland just before the opening of the latter on to the exterior; 

 in Gordiodrilus the external pore of the gland is perfectly distinct from the pore 

 of the sperm-duct but is placed upon the same segment; there are thus various 

 intermediate conditions between the extremes. It can hardly be doubted that these 

 latter genera possess organs which are homologous in spite of the rather different 

 relation of the sperm-duct to them : in the Eudrilidae themselves the position of the 

 opening of the sperm-duct into the gland varies in different genera. To insist 

 upon a difference between the spermidueai gland of the Eudrilidae and that of the 

 Acanthodrilidae, because in one there is a direct connexion between it and the 

 sperm-duct, and in the other there is not, seems to me from a consideration of the 

 intermediate stages to be absurd; and if we apply this argument to the aquatic 

 Oligochaeta the absurdity is even more apparent. In the genus Branchiura which 

 I have recently described, the 'atrium' is appended as a diverticulum to the sperm- 

 duct ; the two open in common, but the ' atrium ' does not receive the sperm-duct 

 at its summit, but at the point where it passes into a muscular duct which leads to 



