OCR Text |
Show 144 DR. W. B. BENHAM ON [Feb. 16, at the tip ; and projecting from this pit is a smaller whitish papilla provided at its apex with a minute pore (Plate VII. fig. 7). I expected, at first sight, that the hinder pair of the papillae would be in connection with the sperm-ducts, but no such relation exists. Each of the four papillae is represented internally by a rounded or kidney-shaped swelling (Plate VIII. fig. 9), from the centre of which (or from the hilum as the case may be) there passes to the body-wall a bundle of fibres (m). These are muscle-fibres and surround a " chaetophore " or sac containing the chaetae, which replace the ordinary ventral chaetae, from which they differ only in their greater length. The papilla itself has the following structure (which is closely similar to that of the next species, of which a figure is appended (Plate VIII. figs. 10, 11): the pore at the apex of the inner papilla (pap.) leads into a sac lined by columnar cells (ep.) forming a definite epithelium ; the lumen of the sac extends in an irregular way for some distance all round the aperture, and its epithelium is, at places, considerably folded. Outside the epithelium are numerous bundles of muscle-fibres (mus.), some radially arranged, some circularly, and some longitudinally (as seen in a transverse section of the body-wall passing through the papilla). These fibres can be traced into the muscular layers of the body-wall, from which they are evidently derived by its invagination. Outside the muscular coat-though not separated from it by any marked line such as the figure suggests-is a thick coat of clitellar cells arranged in groups (fig. 11, gl.). Blood-vessels ramify between the groups and amongst the muscle-bundles. Surrounding the whole is a layer of flattened coelomic epithelial cells (co.ep.), which dips down between the groups of clitellar cells. The muscle-fibres are found only near the aperture in that part of the organ which forms the external papilla; in the remainder of the gland the clitellar cells abut immediately upon the epithelial cells. There is a remarkable resemblance in structure between this organ -which is evidently copulatory in function and capable of slight eversion-and the prostates (or atria) of Perichceta, AcanthodrUus, Trigaster, &c.; the epithelium, however, is more definitely marked off from the gland-cells than in these, and recalls, rather, the prostates of Pontodrilus; or if we compare the structure of the organ near its pore with the atrium of Moniligaster we shall see a still greater resemblance, except that in the latter genus there is, according to Beddard's description \ no membranous coelomic epithelium, for the " clitellar cells " represent this layer. Amongst the families Rhinodrilidce, Geoscolecidce, and Lumbri-cidce, a " prostate " is not usually recognized as being present, but in several genera there is a more or less conspicuous swelling of the body-wall, which is perforated by the sperm-duct in its passage to the exterior. In Geoscolex, Brachydrilus, Criodrilus, and Calli-drilus such structures exist; of the histology of these, however, 1 Q. J. M. Sc. xxix. pp. 119 &c. pi. xii. fig. 11. |