DEVELOPMENT OF CESTODES. 51 



Tapeworm examined. The eggs are round or oval and small. 

 The shell of the egg may be composed of several thin membranes 

 or of a thick and strong capsule. The curious six- or four- 

 hooked embryo (fig. 14, b) may be seen in the most highly 

 developed ova. In one genus of Tapeworms, known as Both- 

 rioeephalus, of which a species is found in the human being in 

 Switzerland and elsewhere, the development takes place partly 

 in the water, the embryo leaving the ovum as an oval ciliated 

 larva. As a rule, we find that a complete metamorphosis exists, 

 which is often connected with a very complicated " alternation of 

 generations." The subsequent stages live in different localities, 

 finding the necessary conditions in different animals or "hosts," 

 between which they migrate either actively or passively. 



The ripe proglottides break off from the worm and are passed 

 out of the host in its excreta, where they remain on dunghills, 

 on the grass, and in water. The proglottides burst by 

 cadaveric decay, and the countless number of contained ova 

 are disseminated over the surface of the earth and in the 

 water ; here they remain until they are taken into the body 

 of some herbivorous, omnivorous, or, more rarely, carnivorous 

 animal. 



"When in the stomach the gastric juice dissolves the capsule 

 of the ovum, and the round embryo is set free with its six, or 

 rarely four, hooks. These hooked embryos bore their way into 

 the gastric and intestinal vessels, when they enter the vascular 

 system and are carried along passively by the flow of blood, and 

 then pass by way of the capillaries to the lungs, liver, muscles, 

 brain, &c., of the animal that has had the misfortune to ingest 

 the ova. The embryos then take up their abode in some organ 

 or connective tissue of their host, lose the hooks, and become 

 converted into cysts, hydatids, or " water-hags,'' which grow into 

 large vesicles with liquid contents. These vesicles become in 

 time what are called Bladder-worms, by the formation of one or 

 more hollow buds which are developed from the walls of the 

 cyst, and which project into its interior. The armature of the 



