A NEW MAMMALIAN CESLODE. 1045 
represented in text-fig. 3, the walls of the tube are thin but 
apparently still cellular. From this dilated extremity springs 
the fine and narrow distal region of the vas deferens, in which 
the actual walls are to be recognised with difficulty. These 
appearances suggest that we have to do here with an extremely 
exaggerated form of a vesicula seminalis, which possibly serves 
other purposes beside the mere storage of sperm; but as to what 
these functions are I have no reasons to form an opinion. It 
is, however, perhaps to be associated with the development of 
the sperm, which does not appear to come to maturity within 
the testes, as I have already pointed out. In all proglottids the 
dilated and glandular region of the vas deferens appeared to be 
full of sperm. 
The cirrws-sae of this worm is large and reaches inwards 
across the two longitudinal tubes of the water-vascular system. 
It is perhaps half a millimetre in length. As already mentioned, 
the cirrus sacs alternate in position with absolute regularity in 
successive proglottids. Each cirrus-sac opens on to the exterior 
in mature segments rather behind the middle of the proglottid, 
and its position is oblique—the posterior end being directed 
anteriorly; the external aperture is thus directed vather back- 
wards. I observed no individuals im copula, and it is dificult to 
understand how this takes place. In the case of other Acoleide, 
which are all, like the present genus, without female orifices in 
the mature seoments, it has been suggested that the spiny cirri 
perforate the body. Of any such spines there seems to be no 
trace in the present species. But frequently the cirrus within 
the sae was dilated with masses of sperm, conspicuous on account 
of its deeply staining with hematoxylin. The cirrus-sac projects 
into a fairly deep genital cloaca which is, in the most anterior 
segments where the cirrus-sac is immature, quite as deep as the 
cirrus-sac is long. The distinctiveness of the genital cloaca is 
rather lost in the mature proglottids. The cirrus-sac itself is 
oval in form, with a gradually decreasing anterior region which 
projects into the genital cloaca. It is, in fact, pear- shaped with 
rather a long stalk. 
When the cirrus-sac is quite fully formed, it is seen to le in 
a rather specialised region of the body parenchyma. This region 
is formed of very lax tissue, which might thus be supposed to 
allow of greater freedom on the part of the cirrus-sac. More- 
over, isolated but numerous muscular fibres run inside and outside 
of this lax area which are attached to the cirrus-sac and probably 
serve as retractors. This lax area is not limited to the region 
occupied by the cirrus-sac alone; it runs back and accompanies 
the first part of the sperm-duct. In a series of sections the 
cirrus-sac and the first part of the sperm-duct are seen to he 
loosely in the lax tissue which extends beyond it in every 
direction. The cirrus-sac is extremely muscular, the walls being 
unusually thick. The muscle-layers are two, the cutside being 
of fibres having a longitudinal direction, and within this is a 
