OCR Text |
Show 1893.] THE GEOGRAPHICAL DISTRIBUTION OF EARTHWORMS. 733 On careful dissection the supernumerary gill (br.s.) was seen to be externally confluent with the prolonged lip of that in front of it (the normal sixth one), while internally its orifice approximated most nearly to that of the cesophago-cutaneous duct. As a whole, it was disposed transversely, instead of obliquely backwards as are the normal gills ; its presence had slightly disturbed the symmetry of the fifth and sixth pairs of gills and the cesophago-cutaneous duct, and its relations were such that it might well have been derived from either that structure or the sixth gill. Unfortunately, the ventral aorta had been so far dissected before the specimen came into Prof. Howes's hands, that it was impossible to follow out the clue which the afferent branchial vessels might perhaps have given to the origin of this extra gill. On minute examination, the external prolongation of this was found to contribute the major share to the adjacent exhalant passage, and to receive rather than merely unite with the gill-passages in front. In this it resembled the ductus cesophago-cutaneus. There can now be little doubt that in Giinther's Bdellostoma cirrhatum we are dealing with a species in which the gills are individually variable from 6 to 7 on either side; and in view of the undoubtedly less specialized condition of the branchial apparatus of this genus than that of Myxine, there was reason for suspecting that the appearance of a seventh gill on the right side in the latter might be a reversional variation, and if so, that the supernumerary gill of the left side might be of a similar nature, and the cesophago-cutaneous duct sui generis distinct from it, if not from the true gills in general. To this view he himself inclined. The specimens exhibited were the only ones out of at least a hundred of both species examined in which the branchiae were thus aberrant. The following papers were read :- 1. On the Geographical Distribution of Earthworms. By F R A N K E. B E D D A R D , M.A., F.R.S., Prosector to the Society. [Eeceived November 21, 1893.] In m y forthcoming Monograph of the Oligochaeta I propose to attempt a general survey of the distribution of the terrestrial forms; the present communication is an abstract of the chapter on that subject. I allow 69 genera of Earthworms, which are distributed as follows:- PAL^ARCTIC REGION. (Fam. LUMBRICIDJE.) Lumbricus. Allolobophora. Allurus. TetragonuruSs |