DEVELOPMENT OF ENTOZOA I OLENIA 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- 

 torna (known under the name of fluke) that infests 

 the livers of Sheep is a characteristic example, 

 undergo a yet longer succession of changes j these 

 have been especially studied in a species which 

 infests the Lymnceus, one of the Water-snails, and 

 which is represented in fig. 308. From the egg 

 deposited by this Distoma is produced a long flat-bodied embryo 



Fig. 307. CYSTICERCUS ; a, head 

 greatly enlarged. 



