OCR Text |
Show 112 MR. F. F. LAIDLAW ON POLYCLADS [May 26, epithelium lies a thick basal membrane which shows no trace of nuclei. On the ventral side the muscles of the body-wall consist first of a very narrow outer layer of longitudinal fibres lying against the basal membrane. These are succeeded by an outer diagonal layer-, which in turn is followed by a few circular fibres, and these by an inner diagonal layer, the fibres in this latter running at right angles to those of the outer diagonal layer. Lastly comes the thick inner longitudinal layer. On the dorsal surface there are first a fine outer longitudinal layer, next a diagonal lajrer, and lastly a thick circular layer. Hence the dorsal muscles are somewhat similar to those of Cestoplana, the ventral are more highly developed (see Lang [1], t. 16. fig. 1). The dorsal tubercles are more muscular and broader in proportion to their height than those of Cycloporus. They are covered with an epithelium, which about their base is similar to that of the rest of the surface of the body, but which towards their apices becomes flattened and loses its pseudorhabdites. Under the basement-membrane, which becomes somewhat attenuated on the tubercles, is a special layer of circular muscle-fibres. In the centre of each tubercle is a small quantity of tissue richly supplied with nuclei, and connected with the parenchyma of the body by a strand of tissue which pierces the muscles of the body-wall. This tissue is perhaps nervous in character (PI. IX. fig; 6, %, lj\ The pharynx is large and much folded. The " mouth " lies behind its posterior end, and communicates with the elongated pharyngeal pouch by a narrow channel. The gut-branches are numerous and lie at different levels (PI. IX. fig. 6, g). Genital Organs. (Plate IX. fig. 7.) Owing to an unfortunate accident some of the sections in the region of the body where these organs occur have been lost. Consequently the account given below is necessarily incomplete. There appears to be a single genital atrium. The penis is provided with a true penis-sheath (cf. Cestoplana), and there is also a prostate gland. The vagina runs backwards and upwards for some little distance, then bends forwards and downwards receiving on its dorsal side the common opening of the uteri. Beyond this it runs forwards and downwards towards the atrium (possibly opening into it ?). M y chief reasons for placing this oval form in the neighbourhood of Cestoplana are to be found in the shape of the penis, the presence of a prostate gland, the backwardly-directecl mouth-opening, and the disposition of the eye-spots. The thickness of the basement-membrane and the arrangement of the gut-branches and muscles of the body-wall are, I believe, indications which point in the same direction. |