OCR Text |
Show 1892.] NEW SPECIES OF EARTHWORMS. 137 species as the other two, but I find that it is an Enchytrseid, the anatomy of which, however, I have not yet worked over. Of the two other specimens one remains entire, and was returned to the British Museum, the drawing of the external surface (Plate VII. fig. 1) being taken from it; the second was partially cut into sections-after being opened and examined-the anterior twenty somites being cut sagittally, the posterior portion transversely. The chief characters of the worm may be summed up as follows :- (1) The eight chcetce are isolated, though they are not all equidistant. (2) The clitellum is complete, and occupies somites xiii. to xviii. (3) The male pores are on somite xviii. (4) The nephridiopores alternate in position, one series being in line with the chsetse "3," the other with the chsetae "4." There are (5) one pair of testes in somite x.; (6) one pair of sperm-sacs in somite xi.; and (7) four pairs of spermathecce, without diverticula, in somites vi., vii., viii., and ix. (8) The cylindrical prostate lies in somite xviii. (9) The gizzard occupies somite v.; there are no definite oesophageal diverticula. The Earthworm which appears to agree most clearly with this diagnosis is Plutellus heteroporus x from Pennsylvania; but this worm, according to Perrier's description, presents two very striking anomalies, which do not occur in the present instance : (a) the nephridia are entirely confined to one somite, i. e. the funnel does not perforate the septum ; (b) the " ovary " is placed anteriorly to the testes. But these two peculiarities are anomalous, not amongst Earthworms only but amongst all the Oligochceta ; the post-septal position of the nephridiostome is indeed totally at variance with the arrangement met with throughout the whole group of Chcetopoda ; hence, we must look with very great suspicion on these supposed characters, and indeed Perrier himself, in writing of the presumed " ovary " in the tenth somite, recognizes its abnormal position and expresses himself, not only with great caution, but also with a good deal of doubt-"Mais nous devons dire qu'a cet egard notre conviction est loin d'etre aussi complete qu'en ce qui concerne les testicules." Most zoologists working on this group have thrown doubts on the accuracy of these supposed facts, and this without impugning the carefulness of M . Perrier, for he had two specimens only, and these, having lain in spirit for 50 years and more, were in a very bad condition of preservation ; moreover, his statements were drawn from observations on the dissected specimens, which are not so likely to be correct as those obtained from examination of serial sections. W e must then remove these two characteristics from the diagnosis of the genus Plutellus, and thereby we bring the worm into accord with what has become regarded as the normal condition of things. I believe the worm which forms the subject of this communication 1 Ed. Perrier, Arch, de Zool. Exper. ii. 1873. |