PLATYHELMINTHES 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 

 , behiiid 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. 



