ANATOMY OF TANIA MEDIOCANELLATA. 11 
(fig. 1, f; fig. 2,7). The merging of its walls into the uterine 
walls does not ensue on that side of the cavity directed 
towards the edge of the segment in which the genital pit is 
situated, and consequently in the most convenient spot to the 
prior course of the vagina, but on the side of the uterus cor- 
responding to one of the flat surfaces of the segment, and hence 
at right angles to its prior course (fig. 3, f), and apparently 
(at least in those instances in which I have been able to 
pursue its entire route from commencement to termination) 
on the same side as the genital tubes in relation to their 
crossing the water canal, furnishing an additional indication 
of dorsal and ventral surfaces of the zooid, and approximating 
in character the Bothriocephalus latus, the central orifices in 
direct line from the uterus in the latter parasite being trans- 
ferred in the T. mediocanellata to the side, while the junction 
of the vagina with the uterus is the same in both. The 
vagina from its external aperture for the distance of =1~ in. 
is of the diameter of =1,4 im., has thick walls of circular and 
longitudinal muscular fibre, and a narrow canal lined with 
ciliz (fig. 7, m), corresponding in the two latter details to the 
inner cylinder of the penis, and staining like it in depth of 
colour and hue on the addition of carmine or magenta. Beyond 
this, these thick walls merge into a thin delicate tube closely 
studded with very numerous black granules, which render 
the canal a very conspicuous and easily discerned feature in 
microscopic sections (fig. 12, a). Corresponding in site to 
the centre of the penis, the vagina is dilated ovally and 
fusiformly to 34,5 in. transversely ; this expansion (fig. 11) 
dwindles into the z,'55 in. diameter of the canal generally, 
which is continued until the side of the uterus is reached, 
where there is a second somewhat pear-shaped dilatation of 
gia in. diameter (fig. 12), from which the again contracted 
tube emerges to coalesce after a sudden bend with the uterine 
canal on a slightly prominent part of its wall. Around the 
muscular portion of the vagina there is a sheath formed by 
the inflexion of some of the fibres from the inner surface of 
the skin. The dark granular particles seated in the wall of 
the canal vary somewhat as to extent of distribution ; they 
avoid the muscular portion, but generally commence in the 
succeeding dilatation, occasionally, however, but partially 
studding it as in figure 11; in the central course of the tube 
they are always present, generally also in the bulb near 
uterus, rarely in the short contracted channel emerging from 
it; they are similar to the black particles generally present 
in the suckers of the head of the colony. 
