OCR Text |
Show 1892.] FROM ALGERIA AND TUNISIA. 35 pair, although they lie chiefly in segment iii. in front of the nerve-cord, open on to the exterior between segments i. and ii. On account of the large terminal end sac, which is prolonged on both sides of the aperture, and may be thus said to have a caecum, it is always easy to make out the external aperture. I am therefore able to be confident about this point, which distinguishes the present species from both Microscolex dubius and Microscolex modestus, and allies it with Microscolex algeriensis. The external pore is to the inside, and slightly in front, of seta 3 *. The three strongly developed hearts of segments x., xi., xii. are present in Microscolex poultoni. The generative organs show no great differences from those of other species. As in M. algeriensis, the true vasa deferentia retain their independence until close to the external aperture; they pass a short way beyond the muscular duct of the atrium, and unite to form one tube, which is surrounded with a thick layer of muscular fibres chiefly circular; this tube is quite indistinguishable in its structural characters from the muscular duct of the atrium ; in a section the two tubes cannot be distinguished except by their position. In the thickness of the body-wall, and near to the external pore, they unite. The penial setae have already been referred to. The sperm-sacs are racemose, and occupy the same position as in Microscolex algeriensis and all the other species of the genus. The funnels of the sperm-ducts are larger and more folded than in that species. I could find neither spermatothecae nor egg-sacs. This latter character connects Microscolex poultoni with M. dubius, but it is quite clear from the above description that the species described here is perfectly distinct from M. dubius. The principal differences are :- (1) The fusion of vasa deferentia in M. dubius to form one tube, which opens into the muscular tube of its prostate. (2) The commencement of the nephridia in the vth segment in M. dubius. (3) The absence of any alteration in- the position of the seta on the clitellum in M. dubius. It shows much the same resemblances to M. dubius that M. algeriensis shows to M. modestus. If Dr. Rosa were not so careful a worker as he has proved himself, I should be almost inclined to suspect an identity. The genus Microscolex has been investigated by Rosa and Fletcher. It was first met with in Italy by Dr. Rosa', who, in a later 1 The ventralmost seta on each side is seta 1, the next seta 2, &c. 2 "Microscolex modestus, n. gen., n. sp.," Boll. Mus. Zool. Torino, vol. ii. no. 19 (3 cuts). 3* |