OCR Text |
Show 1891.] EARTHWORM FROM WEST AFRICA. 175 its deficiency in blood-capillaries-perhaps both ; the extreme vascularity and tbe thinness of the walls of the buccal cavity in Libyo-drilus perhaps account for the fact that it is not usually (? ever) protruded during locomotion. The setce are strictly paired. There are no dorsal pores. The clitellum occupies segments 15 and 16 (in some specimens a part of segment 14 also), and is complete, that is it entirely encircles the body. The male reproductive pore is single and median, upon the border line between segments 17 and 18. The orifice is situated upon the summit of a conspicuous elevation of a yellowish colour. The spermathecal pore is also median and unpaired; it lies on segment 13 between the ventral pairs of setse. The oviducal pores occupy what is at present a unique position among Earthworms, viz. on segment 15. Thus another of the characters supposed to distinguish the group Terricolse no longer holds universally. The chief characteristic of the family Eudrilidse is the remarkable development of ccelomic sacs which enclose the different parts of the female reproductive organs. Libyodrilus, though in some respects approaching Hyperiodrilus, offers a new type. A large sac occupies the dorsal region of segments 14-18; it is closely adherent to the dorsal vessel and oesophagus ; this sac gives off three pairs of approximately corresponding diverticula; anteriorly it divides into two, and embraces the oesophagus as in Hyperiodrilus ; the two parts are reunited immediately below the oesophagus and run forwards and downwards until they reach the nerve-cord ; here they again divide and reunite upon the ventral side of the nerve-cord, to open by a single median orifice upon segment XIII. The oviduct passes from this sac on each side straight to its external orifice. In transverse section it may be seen that the oviducal funnel opens into the interior of an egg-sac ( = receptaculum ovorum, auctorum) which is quite independent of the large spermathecal sac, though lodged within it. The ovaries are only visible in immature worms; they occupy the usual position in segment XIII. The sperm-ducts, which open by ciliated funnels into segments x. and xi., opposite to the testes, retain their distinctness until the point of opening into the atria. The atria have a very thick muscular coat; they open by a common orifice on to the boundary line between segments xvn. and XVIII. ; each is provided with a sac containing a single short penial seta, not ornamented, and ending in a blunt rounded free extremity. The nephridia are paired, but are connected with an integumental network of tubules opening on to the exterior by numerous pores. The alimentary tract consists of the usual divisions ; the oesophagus has no calciferous glands nor ventral pouches ("Chylustaschen " of Michaelsen), which are so characteristic of the Eudrilidae. There |