112 



OLIGOCHAETA 



Fig. 30. 



and Libyodrilus arc among the genera whose spormiclucal glands are 

 furnished with very thick muscular walls ; the fibres are arranged in two layers ; 

 the outermost, which is the thinnest, consists of longitudinally running fibres ; it is 

 these which give the nacreous appearance to the organ ; inside the longitudinal is 

 a thicker layer of circular fibres. In Nemertodrilus the muscular 

 layer is greatly reduced, but it still consists of the two layers 

 of fibres. 



The position of the opening of the sperm-ducts into the 

 glands also varies considerably in different genera ; they never, 

 however, open either independently of them or into the terminal 

 bulbus. The apparent opening is nearly always different from 

 the real opening. For instance in Eudrilus (cf. woodcut, fig. 

 30) the two vasa deferentia pierce the walls at a point which is 

 roughly half-way along the gland ; microscopic sections, how- 

 ever, show that the tubes run side by side between the muscular 

 and epithelial walls of the gland to a point much nearer to its 

 blind extremity, before opening into the lumen. In Libyodrilus 

 the same thing occurs and the actual orifice is near to the very 

 summit. Nemertodrilus occupies the opposite extreme in the 

 series. The sperm- ducts here open into the glands nearer to 

 the external pore. 



The third character of the spermiducal gland in these worms 

 is also subject to some variation. The terminal bulbus is larger 

 or smaller according to the genus. It is very conspicuous in 

 Eudrilus and in Pareudrilus. In the former of these two 

 genera it is a rounded sac which appears on dissection like a 

 hemispherical thickening of the body-wall. From its internal 

 walls arise two processes which, together with the peculiar 

 U-shaped gland connected with one of them, will be referred 

 to under the description of the penis of the Oligochaeta. 



At the opposite extreme perhaps is Heliodrilue, where the 

 bulbus is reduced in size to be merely a slight depression of 

 the external skin into which both the glands open. This 

 terminal sac is protrusible and is generally found more or less protruded in individuals 

 killed with alcohol. 



Finally, it should be mentioned that in Eudnloides Irunneus, at any rate, tracts 

 of epithelium in the glands are ciliated. 



EUDRILUS. MALE 

 EFFERENT ORGANS. 



f. d. Sperm-ducts. pr. 

 Spermiducal gland, a. Ap- 

 pendices of terminal mus- 

 cular sac. 



