406 
Tetrarhynchus erinace us 
anterior to the genital aperture, and the iiterus is then a straight tube 
of relatively narrow diameter, occupying the axial region of the segment. 
Presumably the testes are then in the actively functional stage, and 
copulation between the segment containing them in this phase and 
other segments may take place. But it seems to be quite probable 
that simple emission of the semen into the surrounding sea water may 
occur, and that the spermatozoa may be taken into the vagina from 
outside the segment. Self-fertilisation (but not self-copulation) may 
indeed occur but the mass of sperms contained in the testes in a ripe 
proglottis in the male phase is so great that it does not seem possible 
that all can be contained in the vagina. Some loss is therefore 
probable and it is likely that this occurs as the eggs ripen and are 
accumulated in the uterus. 
In a younger stage than that represented in Fig. 8 the uterus is 
still smaller relatively to the other organs. It is roughly egg-shaped 
in transverse section, and has a restricted lumen and very thick walls 
(PI. XXII, fig. 16). The basis of the latter is a structureless membrane of 
the usual type, inside which is a single layer of cells, and outside about 
four to six layers of cells. The basement membrane bears externally (in 
transverse section) a number of little projections which appear to be 
the cross sections of longitudinal fibres—these disappear in later stages. 
The internal nuclei ai'e embedded in a matrix which appears to consist 
of broken down cell-bodies—certainly no cell outlines are recognisable 
and the surface facing the lumen is a broken and irregular one. The 
external cells are, on the contrary, very distinct: they are pear-shaped 
and the stalks run towards the basement membrane, though they cannot 
be traced into the latter. The nuclei are large and distinct and have 
prominent nucleoli. The whole wall has a kind of glandular appearance 
rather like that of the shell gland, but this appearance is illusory since 
the staining reactions of these cells are not, at any period, those of 
actively secreting structures. There can be little doubt that they are 
the formative blastema of the uterus itself, and that they become 
transformed into its structureless wall as the organ increases in size by 
the reception of the fertilised eggs. 
The vagina. (Pis. XXI and XXIV.) This is a fairly wide and thin 
walled tube running back from the genital aperture. The latter it will 
be remembered is a wide, shallow pit situated on the postero-lateral 
margin of the proglottis. From the floor of this pit a canal takes 
origin—the common genital canal—and this soon branches, one branch 
becoming that part of the vas deferens which is situated within the 
