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Show 110 MR. F. F. LAIDLAW ON POLYCLADS [May 26, tomoses, but it is difficult to determine this owing to the great accumulation of eggs in the lateral parts of the body. The "mouth" is about PS m m . behind the anterior margin. The opening of the antrum masculinum is about 1 m m . behind that of the " mouth." It is very minute. The small vasa deferentia are full of spermatozoa; at the level of the antrum masculinum they become much contorted, and finally both open into a very small median vesicle, which latter appears to open directly by a minute pore into the antrum. Thus there would appear to be no intromittent organ, though it is quite possible that the vesicle may be to some extent everted. The vesicle is provided with a very thin wall of circular muscle-fibres. The antrum femininum opens some -25 m m . behind the male aperture; it is small and surrounded by a number of shell-glands. Dorsally it passes upwards and then backwards, receiving as it turns back the common opening of the two uteri. Beyond these it is prolonged into the very small accessory vesicle. The uteri extend along either side of the body not far from the middle line ; they vary much in diameter in different localities, and in places contain spermatozoa. At intervals, where they become dilated, the eggs can be seen making their way into them by what appear to be simply gaps in the uterine walls. The whole lateral regions are crowded with large eggs, which lie embedded in a matrix that consists apparently of a yolk-like material (PL IX. fig. 4). The female aperture is of such small size that it seems impossible that the eggs can escape through it. Possibly the body of the parent ruptures after a time and allows the eggs to pass out. This view is suggested by the fact that in some of the sections eggs can be seen lying in the gut itself. The genus may be defined as follows:- Body small, oval, and rather stout. No tentacles or sucker. Body-wall muscles feeble. The pharynx of a simple type, opening at the end of the first fourth of the body. Male genital apparatus of small size, copulatory organ much reduced. Female apparatus simple, with a small accessory vesicle. Eyes in two rows over the brain. The position of this curious form amongst the Leptoplanida? depends on negative rather than on positive characters. Its exact affinities are doubtful, and it is probably a degenerate organism. Family CESTOPLANID^E. CESTOPLANA FILIFORMIS, sp. nov. " 15.2.01. Ribbon-shaped, about 1 in. x £ in. Creamy white with bright yellow border, and a median stripe of the same colour " (cf. C. rubrocincta for colour). The " mouth " lies within T 5 m m . of the hinder extremity. Evidently closely allied to the Mediterranean C. rubrocincta, |