PLATVHELMINTHES 99 
rise to some nerves which pass forward and supply the suckers, 
and to two stout nerve cords which pass back along each side 
of the proglottides, lying just outside the longitudinal ducts of 
the excretory system (Fig. 67). 
Both male and female reproductive organs are repeated in 
each proglottis (Fig. 66). The testis is composed of very 
numerous vesicles in which spermatozoa arise, and which are 
each attached to one of the branches of the ramifying vas 
deferens. Thus the testis is dispersed all over the proglottis. 
The branches of the vas deferens unite and form a common 
tube, which is slightly coiled, and which runs from the centre 
of the proglottis to the common genital pore situated in the 
middle of one side (Fig. 66). The portion of this common vas 
deferens lying next the orifice has very muscular walls, and 
can be extruded. It functions as a penis. 
Self-fertilisation takes place, and is probably the usual 
method, there is no evidence to show that one proglottis 
fertilises another. 
The vagina opens into the genital pore a little behind the 
penis, it then passes backward to a small swelling, the 
receptaculum seminis. In this the spermatozoa are stored 
until the ova are ripe for fertilisation. The ovaries are two in 
number, and are composed of numerous tubules on a branch- 
ing duct. Each ovary gives off an oviduct, and the two 
unite, and then receive a small duct which comes from the 
receptaculum seminis and conveys the spermatozoa ; it is there- 
fore called the fertilising canal (Fig. 66). The oviduct then 
receives a duct from the yolk gland, which lies between and 
behind the ovaries, and passing through a small spherical 
shell gland, which deposits the shell, enters the uterus. 
The uterus is at first an inconspicuous simple sac, whose 
only opening is that from the oviduct. As it becomes full of 
eggs it increases greatly in size, and becomes much branched ; 
eventually it occupies almost the whole of the interior of the 
proglottis, the reproductive organs of both sexes having atro- 
phied (Fig. 66, F). A proglottis which is ripe for separation 
consists of little more than a sac—the uterus—crowded with 
minute spherical eggs, which eventually escape by the rupture 
of its walls. 
