DEVELOPMENT OF CESTODES. 45 



As a rule, we find that a complete metamorphosis exists, which is 

 often connected with a very complicated " alternation of gener- 

 ations.'' 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 herhivorous, omnivoruus, or, more rarely, carnivorous 

 animal. 



When in the stomach the gastric juice dissolves the capsule 

 of the oviim, 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 then- host, lose the hooks, and become 

 converted into cyds, hydatids, or " water-haffs^' which grow into 

 large vesicles with liquid contents. These vesicles become in 

 time what are called Bladd&i'-worins, 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 

 future scolex is developed at the bottom of these invaginations. 

 By a process of evagination we see that the true scolex and neck 

 are found attached to the bladder or vesicle. 



Varieties of cysts. — When only one invagination is formed, 

 the cyst is known as a Cysticercus (fig. 11, a); when several 

 are produced, as a Cwnurus (b) ; and when the cyst itself be- 

 comes invaginated, and each of these invaginations gives rise 

 to smaller invaginations (scolices), the structure is called an 



