OCR Text |
Show '887.J ANATOMY OF EARTHWORMS. ^77 greater portion forms a tuft situated in the neighbourhood of the ventral pair of setae (c), between these and the nerve-cord; the distal extremity of the tubule opens by the ordinary funnel-shaped aperture (cr) into the next segment in front. The excretory tubule then widens out, and forms a section (b) which presents a close structural agreement with that lettered (d) in m y figures of the nephridia of Thamnodrilus1; this opens into the distal section of the organ (a) which runs paiallel with the last ; its walls appear to contain a few muscular fibres. A histological account of the several regions of the nephridium is deferred for the present. § Generative Organs. Female Generative Apparatus.-I have investigated the structure of the female generative organs by dissection as well as by means of transverse and longitudinal sections through the legion of the body which they occupy. The excellent state of preseivation of the specimens enables m e to add some few facts to those which I have already published2 concerning the anatomy of the female reproductive organs. I have also been able to observe some facts beaiing upon the development of the ovarian ovum, but these I propose to make the subject of a separate paper. The main facts with respect to the anatomy of the female reproductive organs stated in m y former papers, I am able to confirm from the study of the present species. In E. sylvicola as in E. boyeri (and probably in all other species) the ovary is continuous with a much-coiled duct with ciliated lining epithelium and muscular walls (Plate XXXIII. fig. 12) ; this opens on to the exterior in common with a large spermatheca, upon the duct of which a small gland is sessile. A dissection of the present species shows that the female reproductive organs, although opening on to the exterior in the 14th segment, occupy both this and the 13th segment ; the mesentery between the two segments is apparently absent. An examination of a series of longitudinal sections shows that the mesentery is not entirely aborted ; the large spermatheca lies in both segments, and the mesentery dividing them is attached to the sides of the s-perma-theca; lower down (tee woodcut, fig. 1, p. 381) the mesentery, when present, divides off the ovary, which lies in the 14th segment, from the oviduct, the greater part ot which lies in the 13th segment, and fiom the glandular diveiticulum of the speimatheca which lies in the same segment (sp, fig. 1). Since the female generative aperture lies in the 14th segment as well as the ovary, it is clear that the oviduct, the greater part of which lies in the 13th segment, must perforate the intersegmental mesentery twice. It is clear from m y sections that the relative positions of the ovary and its duct are precisely the reverse of that which is indicated in 1 Woodcuts, figs. 5, 6, P. Z. S. 1887 (pp. 160, 161). 2 Zool. Anzeiger, Bd. ix. p. 342 ; P. Z. 8. 1886, p. 202 ; Proc. Roy. Soc. Edinb. uu. 122, p. 6. |