198 



AN INTRODUCTION TO ZOOLOGY 



u. 



u. 



yolk gland, is surrounded by the shell gland, and then passes forwards 

 as a large median closed sac, the uterus. The yolk or vitelline gland 

 is a conspicuous structure, composed of two lobes lying across the 

 posterior end of the proglottid and opening into the oviduct by a 

 single median canal. The shell gland, situated around and covering 

 the bend of the oviduct, is a more diffuse gland without any special 

 duct. The external female aperture leads into a fairly wide straight 

 tube, the vagina, which passes obliquely inwards, and, as just noted, 

 opens into the oviduct a short distance from its commencement. 

 x The inner end of the vagina is swollen out to 



form an oval seminal vesicle. 



Copulation has not been observed in T. 

 solium, so that it is not known for certain 

 whether the proglottid is self -fertilising or is 

 fertilised by another proglottid. In view of 

 the fact that in certain allied species only one 

 mature proglottid is present at a time, it seems 

 probable that the former is the case. The 

 sperms pass down the vagina and are stored 

 in the seminal vesicle. The ova shed from the 

 ovaries pass along the oviduct and are fertilised 

 as they pass the entrance of the vagina. As 

 they proceed further they become surrounded 

 by a layer of food-laden cells from the vitel- 

 line gland, and then around each ovum and 

 its attendant cells is secreted a thick chitinous 

 shell, the product of the shell gland. These 

 ova then pass on to the uterus, -where they 

 are stored and where they undergo a certain 

 amount of development. The production of 

 more and more eggs leads to the degeneration 

 of the gonads and the filling up of the uterus. This is at first a 

 simple sac, but as it fills it sends out lateral branches until it comes 

 to occupy practically the whole of the proglottid, which is now 

 termed ripe and is ready to be passed to the exterior with the faeces 

 of the host. The form assumed by the branched uterus in the ripe 

 proglottid is quite characteristic, and is often a valuable guide to the 

 species of tapeworm. Sometimes when passed out in this way the 

 proglottid is capable of moving about for a short time and may 

 be swallowed whole, or more frequently it disintegrates, releasing 

 the ova. The intermediate host in the case of T. solium is the pig. 



While still contained in the uterus in the proglottid the egg 

 undergoes its segmentation and grows into a tiny spherical body 

 covered with a cuticle and provided with six chitinous hooks, and 



FIG. 63. Ripe pro- 

 glottid of Tcenia 

 solium. 



G., genital aperture ; U., 

 uterus full of eggs. 



