OCR Text |
Show 1906.] WORMS OF THE THIRD TANGANYIKA EXPEDITION. 213 found " swarming in great numbers round roots of water-weeds in shallow water." It will be obvious in the course of the following description that the worm is either rightly referred to this genus and subgenus or that it requires a new genus or subgenus for its reception on account of certain peculiarities which will be duly noted. A specimen which I have selected as the type (as regards external characters) measured 38 m m . in length and consisted of 96 segments. The setce appear of considerable length in proportion to the diameter of the body. They are of the usual shape, but distinctly bifid at the tip, though it often happens that the upper half of the cleft extremity is worn down and the seta thus appears to be merely hooked. I believe that the existence of uncinate setae is new to this particular group of Oligochaeta. The clitellum is not very extensive, occupying as it does segments xiv.-xviii. and commencing or ending, as the case may be, towards the middle of each of these segments. The clitellum is saddle-shaped. The generative pores, the actual orifices, are not very plain on the mounted specimen. But from serial sections I have ascertained that the spermathecal pores lie between segments viii./ix. and the male pores upon segment xvii.; the latter nearly in line with the ventral setae, and the former near the lateral setae. It is to be noted that both setae of the ventral as well as the dorsal pair are present upon segment xvii. and that they are not in any way modified. The male pore on each side is just to the outside of the pair of setae, and is borne upon a prominent flap which is not invaded by the clitellar epidermis. Its structure will be dealt with later. The alimentary cay ted is without a gizzard. In the ixth segment the oesophagus is provided with a ventral pouch, which whether single or paired is so characteristic of the subfamily Ocnerodrilinae. In the present species, however, this pouch, which is single, is greatly reduced in size and bifurcates into two after its emergence from the gut. Indeed, if it were much larger there would be, in view of the large size of the spermathecae, hardly room for it in the ixth segment. It is a smallish sac lying ventrally to the oesophagus and narrowing at its junction with the oesophagus very anteriorly in the ixth segment. It has not a specially glandular appearance, and the lining epithelium is merely folded. There is no such complicated folding as occurs, for example, in Gordiodrilus. The ventral pouch of this species appears to be either an incipient or a degenerating structure. A largish blood-vessel is attached to the posterior end of each bifurcation. The septcd glands of the present species extend back into the viith segment. The vascular system is noteworthy on account of the extreme vascularity of the integument, which is equally obvious in the specimen mounted entire and in sections. This was especially plain in the anterior region of the body. If the capillaries do not actually penetrate the epidermis, they only cease just below 15* |