| OCR Text |
Show 32 MR. W. K. HUTTON ON THE [Jan. 20, Around the anus and the external nephridial apertures, both layers furnish special sphincter-fibres ; while along either side of the nerve-cord, as is afterwards noted, there runs a slender longitudinal strand of muscle-fibre (N.C., fig. 6). The general arrangement of the four special retractor muscles of the introvert and of the radial rectal muscular attachments have already been described ; there remains only, in regard to the former, to speak of the special appearance presented by the muscular fibres in this worm. On comparing the ventral retractors with the general longitudinal musculature, one is struck by the great similarity of texture presented by both. A glancing, bright suggestion of firmness, reminding one of the surface of fresh tendon among the higher forms, is noticeable. The dorsal retractors, on the other hand, have a dull, almost a translucent appearance. Microscopical examination of transverse sections reveals little difference between the two sets of muscles, save only that in a given area fewer muscular fibres are present in the dorsal muscles, and the amount of intermuscular connective tissue appears to be greater than in similar sections of the ventral set (Plate VII. fig. 7, A & B). On the whole, both from the relatively marked shortness of the dorsal muscles and from the appearances just described, I incline to suspect that they cannot be functionally very active. They contain a sufficient amount of connective-tissue between the proper muscular bundles to make the suggestion that they are in a state of metamorphosis and regression at least probable (vide J. Bland Sutton's ‘ Ligaments,' <fcc., 2nd ed., 1897, p. 3 et seq.). Alimentary Canal. Surrounding the mouth, which lies at the bottom of the depression formed by the union of their bases, are several simple leaf-shaped tentacles. So far as could be ascertained by the examination of a series of sections, 15 tentacles are present, and of these 11 are longer than the others. Each tentacle contains a branch of the blood-sinus and a slender nerve (Plate V II. fig. 8). This figure, which represents a typical transverse section of a tentacle, shows that on the mesial aspect the cells covering its surface are narrow, long, arranged in palisade form, and are ciliated, while those of the opposite side form a simple columnar epithelium. For the greater part of their extent the tentacles are free, but round the mouth their bases are fused together, and form a series of gutters which lead to the oral aperture. The mouth itself lies on the top of a papilla, whose sides are grooved by the above-mentioned furrows. The cells lining the grooves are cubical and ciliated, and are continuous with the ciliated epithelium on the mesial aspect of the tentacles. Succeeding the mouth is a short cesophagus with very muscular walls; in section (Plate V II. fig. 10) it appears to be star-shaped, the lumen of the tube being encroached upon by the inward projection of a |
