OCR Text |
Show 1901.] AND ARRANGEMENT OF EARTHWORMS. 189 space lying between the ventral most seta of each side. The lateral setae are so closely approximated that they are not more than j of a millimetre from each other. The distance of the lateral pair from the outermost seta of the ventral pair is about equal to that of the ventral median space. The dorsal median space measures 11 m m . The total circumference of the worm is thus about 27 m m. At the tail end of the body the space between the setae of the ventral pair retains its width until the very extremity when it is narrowed. This, however, is simply a matter of the diminished calibre of the body; there is no proportional decrease in the interval. I could find no dorsal pores, which structures appear to be absent in the family Eudrilidce. The nephridiopores are very obvious and lie in front of the lateral setae. The clitellum occupies segments xiii.-xviii. and is completely developed all round the body. Dotted over the segments, and often forming continuous lines with the setae suggestive of a perichsetous condition, are the numerous integumental sense-organs, well known to occur in this genus. In m y Monograph of the Oligochaeta the position of the external generative apertures is accidentally reversed. In Polytoreutus gregorianus, as in all the other species of the genus, the male pore lies in front of the spermathecal pore. The male pore (see text-fig. 50, p. 188) is placed accurately between segments xvii./xviii. It is transversely oval, indeed slit-like, and lies at the summit of a protuberance which occupies the middle part of segments xvii. & xviii. between the ventralmost setae. On the xviiith segment this protuberance appeared to bear two faintly marked papillae, one on either side. The spermathecal pore is on the xixth segment, quite in the middle of that segment; it is rather a larger orifice than the male pore. I append for the purposes of comparison drawings (text-figs. 51, 52, p. 190) of the external characters of the two species Polytoreutus kilindinensis and P.finni. Of neither of these species have the external characters been at present figured', though their internal structure has been dealt with by myself2. Figures of the external characters of the genus Polytoreutus are at present limited to a figure of P. magilensis3. It will be noticed that while P. kilindinensis entirely agrees with P. gregorianus in the position of the male pore and of the spermathecal orifice, P.finni differs in that the male pore is very distinctly in the middle of the xviith segment, and the spermathecal pore is situated on the boundary line of the xviiith and xixth. It will be observed from the accompanying sketch (text-fig. 51) 1 Save for a quite rough sketch of P. finni. 2 " Oligochrcta of Tropical Eastern Africa," Quart. Journ. Micr. Sc. vol. xxxvi. p. 236. 3 " Some new Species &c. of Earthworms," ibid. vol. xxxiv. |