160 



A COURSE ON ZOOLOGY. 



The eggs, after having been expelled from the intes- 

 tine with the matured segment, are carried by the rains 

 into dung-hills and feeding-troughs, where they are swal- 

 lowed by the pig, which animal we know is not delicate 

 in the selection of its food. There the egg, which had 

 begun to develop, gives birth to a sort of larva having 



six little hooks. By the aid of 

 Fio. 93. these hooks the embryo per- 



forates the intestinal wall of 

 the pig, and migrates into the 

 muscles and cellular tissue ; 

 hereitbecomesencysted, that 

 is, completely enveloped by a 

 membrane. It loses its hooks, 

 but soon on the wall of the cyst 

 BLADDER-WORM. appears a little bud, which is a 



head with hooks and suckers, 



and there thus exist in the flesh of such pork little sacs 

 or cysts containing a stalked head fixed to the wall of 

 the sac. 



This form of the tape-worm is called the cysticercus or 

 bladder-worm. It remains in its fleshy enclosure without 

 further development ; but when the pig is killed and its 

 flesh eaten by man, the cysticercus arrives in new sur- 

 roundings, throws off its envelope, fixes itself to the 

 intestine by its head and hooks, and in a few weeks 

 produces a long chain of joints. The creature is now an 

 adult. 



Here, then, is a parasite that to attain complete ma- 

 turity must pass through two different stages in two 

 different animals. This mode of development is quite 

 frequent in animal parasites. For the development of 

 each of these forms, embryonic and adult, a particular 



