114 ZOOLOGY. 
layer is thrown off (this probably corresponding to the serous 
membrane of insects and crustacea); the central mass 
(which is not hollow as in the gastrula of other worms, a 
digestive cavity not being present in after life) forms the 
embryo, and soon three pairs of hooks arise. Three struc- 
tureless membranes are secreted around the embryo, which 
then hatches. The embryo of Bothriocephalus is provided 
with a ciliated membrane, which corresponds to the first 
blastodermic moult of the embryo Tania, which, on the 
other hand, is not ciliated. 
The history of the human tape-worm, Tenia solium (Fig. 
73) is as follows: the eggs eaten by the hog are developed 
in its body into the larval tapeworm (scolex), called in this 
species Cysticercus cellulose (Fig. 75; Fig. 76, head en- 
larged). The head with its suckers is formed, and the 
body becomes flask-shaped ; the Cysticerci then bury them- 
selves in the liver or the flesh of pork, and are transferred 
living in uncooked pork to the alimentary canal of man. 
The body now elongates and new joints arise behind the head 
until the form of the tapeworm is attained, as in Fig. 73. 
The hinder joints then become filled with eggs and break 
off, becoming independent zooids comparable with the 
‘* narent-nurses ’’ of the Cercarias, except that they are not 
contained in the body of the Tenia (as in the Cercaria), but 
are set free. The andependient joint (Figs. 74, 75) is 
called a ‘‘ proglottis.’’ It escapes from the alimentary tract 
of its human host, and the eggs set free, in and about 
privies, are swallowed by that unclean animal, the pig, and 
the cycle of generations begins anew. We thus have the 
following series of changes, which may be compared with 
. the homologous series in the flukes : 
1. Egg. 
2. Morula. 
3. Double-walled sac (gastrula ?). 
4. Proscolex, free embryo with hooks, surrounded by a 
blastodermic skin. 
5. Scolex (Cysticercus, larva). Body few-jointed. 
6. Strobila (Tenia). Body many-jointed. 
%. Proglottis (adult). 
