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Show 1901.] OBTAINED DURING THE " SKEAT EXPEDITION." 85 the body might be vascular if the anterior end was not, since the tail in the Tubificidae seems to be often used for respiratory purposes. But here, as elsewhere, I could find no evidence of blood-capillaries in the skin. A specific difference in a feature of such apparent importance is somewhat unexpected. But, as has been, aud as will be, seen, the present species is in many ways divergent from its congeners. Male organs of generation.-As will have been gathered from the account of the external features of the present species, the segments occupied by the various parts of the male generative system are a segment behind those which are occupied by the corresponding organs in the other two species of the genus. In Bothrioneuron iris the testes are in segment xi. instead of x. Excepting in their position, there is nothing especially noteworthy about these gonads. The male efferent apparatus, as in other species of the genus, is complicated and specialised into a number of regions. In transverse sections of the body the ventral surface was seen to be flattened, and thus to contrast with the semicircular dorsal region. At the sides of the body, the flattened under surface was limited by a slightly projecting ridge, so that the outline of a section was somewhat that of a round hat with a brim also in section. In the middle of this area opens the single pore. W h e n a specimen of the worm is examined in its entirety, the actual orifice is seen to be small and accurately median. In longitudinal sections the smallness of the orifice is also striking. But in transverse sections it appears to be larger owing to the fact that the incurving sides of the body-wall diverge from each other considerably laterally in their course. The relative size of the male pore would seem to be much that of B. vejdovskyanum as figured by Stole But this author does not figure microscopical sections of his species. A noteworthy difference between the two species, which has already been referred to in dealing with external characters, is the total absence of genital setae in B. iris. In this it agrees with its nearest ally B. americanum. It is unlikely that I should have overlooked these setae in two species which have been both of them examined in sections as well as in their entirety mounted in glycerine. There are, in fact, no setae in the immediate vicinity of the male pore. The terminal male apparatus of B. iris is divisible into the same regions as those which are to be found in B. vejdovskyanum • but their relative dimensions are decidedly different, and there seem to be also differences in their histological structure. The sperm-duct is divided into two different sections as in B. vejdovskyanum. The proximal part, that which immediately arises from the funnel, is about as lono- as the region which follows, and is much coiled in the middle of a mass of cells which represents a thickened peritoneal investment. This proximal section is of less calibre, and its cells are equally stained by the borax-carmine used in the preparation of the sections ; the tube is also of less calibre. The remaining part of the vas deferens also coils about iu the midst of the cells mentioned: but |