DEVELOPMENT OF ENTOZOA : TiENIA AND CYSTICERCUS. 567 
embryos escape from them. These embryos, which are small 
vesicles- furnished with six minute hooks or spines, make 
their way through the walls of the stomach into the substance 
of other .viscera; and by getting into the current of the cir¬ 
culation, they are sometimes carried to remote parts of the 
body. Nourished by the juices which it absorbs, the vesicle 
swells : and a head resembling that of the Tape-worm (fig. 
307, a), begins to bud from its wall into its cavity. In this 
condition the product of the egg of the Tape-worm has long 
been known as the Cysticercus (its presence in large numbers 
in the flesh of the Pig giving to it that diseased appearance 
which is known as “ measly ”), without its relationship to its 
parent being in the least suspected; and it undergoes no 
further change until the flesh of the animal it inhabits is 
devoured by some other, so that the Cysticercus is conveyed 
into the intestinal canal of the latter. The head which was 
previously turned into the vesicle, now protrudes from its 
exterior (fig. 307), and attaches 
itself to the intestine of its new 
host by means of the hooks and 
suckers with which it is furnished 
(a); the vesicle is then cast off, 
and its place is taken by the 
series of generative segments suc¬ 
cessively budded-forth from the 
head, which constitutes the body 
of a new Tape-worm; and from 
the ova which these produce there 
springs a new generation, which 
repeats the same curious cycle.—Numerous other parasites 
present a history that resembles the preceding in 
all its essential features, whilst varying in details 
(see Zoology, §§ 925, 926). 
743. The Trematode Entozoa, of which the Dis¬ 
toma (known under the name of fluke) that infests 
the livers of Sheep is a characteristic example, 
undergo a yet longer succession of changes ; these 
have been especially studied in a species which 
infests the Lymnceus , one of the Water-snails, and 
which is represented in fig. 308. Erom the egg 
deposited by this Distoma is produced a long flat-bodied embryo 
Fig. 307.— Cysticercus; a, head 
greatly enlarged. 
