OCR Text |
Show 1897.] FROM SOUTH AFRICA. 347 coloration produced by the blood-vessels. The skin is thin and several of the organs show plainly through. As in nearly all Eudrilidae, dorsal pores appear to be completely absent. The only exception seems to be the genera Platydrilus and Eudriloides. In no other family of terrestrial Oligochaeta is there this nearly complete absence of structures so characteristic of earthworms. The absence of these pores may perhaps be related to the partially aquatic life of at any rate many members of the family ; on the east coast of Africa the species of Eudrilidae are largely met with in swamps, and we know that the purely aquatic genera of Oli-gochaefa are without the dorsal pores for the most part. Exceptions to the statement occur among the aquatic members of the genus Acanthodrilus. But among the " Limicolae" of Claparede there are no exceptions. Another feature in the organization of the Eudrilidae which may possibly be correlated with the absence of dorsal pores, is the often exceedingly dark pigmentation of the chloragogen cells which cover the intestine, and the accumulations of secretory products within the peritoneal cells wdiich cover the nephridia. If the dorsal pores have an excretory function, their absence wrould naturally lead to a greater accumulation of such waste substances as those referred to. That many of the Eudrilidae have a great deal of pigment in the skin may be another fact to be noted in the same connection. But this pigmentation is not more excessive than in worms which possess dorsal pores. The setce have the arrangement that characterizes others of the West African genera that have been mentioned as nearest of kin. The ventral setae are at some distance from each other, while the more dorsally placed pair are strictly paired. The distance which separates the two setae of each ventral pair is five or six times as great as that which intervenes between the two setae of the dorsal pair. The setae themselves present nothing noteworthy in their form; they are rather small and not obvious until the skin is examined with the microscope. The nephrldlopores are very conspicuous orifices lying in line with the dorsal pair of setae. W h e n a piece of the skin is examined after being well cleared with glycerine the tegumentarg sense-bodies, which occur in many Eudrilicls, are to be seen. These structures, which have so curious a resemblance to the Pacinian corpuscles of vertebrates, lie imbedded in the skin here and there apparently without regular arrangement. They are in the present worm of an elongated from and lie invariably with their loug axis corresponding with the long axis of the worm's body. The clitellum, as in many other species, is developed right round the body, having therefore the form to which it has been proposed to restrict the term cingulum. It occupies segments xiii.- xviii. The generative orifices are exceedingly conspicuous. They are unpaired. The female orifice is situated on the boundary line of segments 23* |