402 
Tetrarhynclms erinaceus 
on the duct of the fluid contained in the cirrus sac, so that the floor of 
the genital pit and the terminal part of the cirrus are turned out in 
just the same way as the proboscides are evaginated. Retraction of the 
cirrus is probably brought about by the relaxation of the walls of the 
cirrus sac, and the contraction of the radial muscles which then pull the 
cirrus inwards. I have only seen the cirrus everted on one occasion in 
a living proglottis, when it was situated on the summit of the genital 
papilla, the genital pit being itself then evaginated. It was about 1 mm. 
in length, and there were no hooks or armature of any kind, the outer 
surface of the wall being formed of a clear cuticular substance. 
Since it is the common duct formed by the union of the terminal 
parts of the vas deferens and vagina which is evaginated, it does not 
appear to be possible that self-copulation can take place in Tetra¬ 
rhynclms erinaceus. 
The female organs. The female genital organs in Tetrarhynchus 
erinaceus conform to the usual Cestode type. The vagina jDasses back 
from the genital aperture and joins the oviduct immediately after the 
latter has issued from the ovary. The vessel so formed receives the 
vitelline duct and the efferent parts of the cells composing the shell 
gland, and then runs forward to form the uterus. It will be seen from 
PI. XXI, fig. 10 and PI. XXIV, fig. 11 that the greater part of the 
immature proglottis anterior to t-he genital aperture is taken up by the 
testes, the vas deferens and the vitelline glands ; while the corresponding 
region of the mature proglottis is occupied by the pregnant uterus. 
The other organs are thei’efore restricted to the region behind the 
genital aperture and so great is the crowding here that it is quite 
impossible to make out the relations of the ducts in cleared pre¬ 
parations; and even in serial sections the junction of the various ducts 
is contained within comparatively few sections so that the reconstruction 
is a matter of some difficulty. I have simplified the actual conditions 
in figs. 10 and 11 by making the ducts a little larger than they 
ought to be and smoothing out the coils to some extent, otherwise the 
relations would be exceedingly difficult to represent in a diagram. In 
PI. XXII, fig. 12, however, the various organs are drawn to scale. 
The ovary. This structure (PI. XXII, fig. 12 ; and Pis. XXI and 
XXIV, figs. 10 and 11) is bi-lobed consisting of right and left halves 
joined by a median bridge. It is secondarily lobulated and rather diffuse 
in structure, extending to some extent between the vitelline glands and 
other organs. The lobules are invested by a very delicate membrane 
apparently devoid of nuclei. The structure of the ovary is rather loose 
