OCR Text |
Show 1903.] POLYCLADS OF THE " SKEAT EXPEDITION." 305 inner walls of the prostate chambers. Between each of these chambers a fine double septum runs out from this inner wall (Plate XXIII. fig. 2, s.). The prostate cells stain very feebly, and the lumen of each chamber is well defined, and contains in most cases a certain amount of secreted matter. Outside the chambers is a fairly thick layer of circular muscle-fibres, and, beyond these, an ill-defined layer of cells which appear to be glandular. Here and there are faint indications of processes from these outer glandular cells, piercing the muscle-layer of the prostate organ. This muscle-layer is also traversed by processes from the septa of the chambers. There is no longitudinal muscle-coat to the prostate. Towards the level where the prostate-chambers open into the ductus ejaculatorius, some of them open into each other, so that the number of chambers seen in cross-section is reduced to four or five. On the proximal, hinder, side of the prostate, the ductus is continued back into the vesicula seminalis (v.s.), which is long and contorted. It is lined with a ciliated epithelium, and its walls are composed of regularly arranged circular muscle-fibres. At its extreme hinder end it turns sharply forward and ends blindly. Just before it takes this turn forward, the two vasa deferentia open into it, one on either side, after piercing through its muscular wall. There is a spacious antrum femininum surrounded by the large shell-glands, the secretions of which it receives. The vagina leaves the antrum dorsally, and runs forward for the first part of its course through the shell-glands, the secretion of which it also receives. It then turns first dorsalwards and then backwards, acquiring in its course a fine wall of circular muscle-fibres. After running back for a short distance, it receives the short common duct from the two uteri. It then continues to run back as far as the level of the antrum femininum, where it ends blindly. It is evident that this species is closely allied to von Plehn's Plagiotata promiscua [7]; in fact, the terminal parts of the male apparatus in the two species have almost identically the same structure. It differs from von Plehn's species in having the pharynx of the normal type, not elongated transversely. This character is of sufficient importance to justify the placing of these species in two separate genera. In addition Notoplana is characterised by differences in the female apparatus, and by the great length of the antral chamber of the male organ. The genus Notoplana may be defined, then, as follows:- A Planoceroid genus with styliform penis and without a bursa copulatrix. The male antral chamber is very long; there is a complicated prostatic organ consisting of several chambers lying around the ductus ejaculatorius in front of the vesicula seminalis. The latter is long and twisted. Body rather elongate, without marginal eyes ; mouth-opening rather behind the middle ; pharynx normal: gut-branches numerous, ivithout anastomosis.2 0* |