PARASITISM 



327 



hooks. Very soon each one migrates from the alimentary 

 canal to the muscles, liver, or peritoneum, and forms about 

 itself a sac or cyst, and is then known as a cysticercus. 

 Before any further growth can occur it must reach the 

 alimentary canal of the same species as that in which 

 its parent lived. Tcenia solium, a tapeworm occasionally 

 present in man, passes its larval stage in the flesh of the 

 pig, while the cysticercus of the other human tapeworm 

 is found in beef. In 

 some regions one out 

 of every five rabbits 

 contains the cysticerci 

 of the dog's tapeworm 

 in the liver or perito- 

 neum. They are oval 

 white bodies from a 

 fourth of an inch to 

 half an inch in diam- 

 eter. It is also quite 

 easy to find the cysti- 

 cerci of the cat's tape- 

 worm in the liver of 

 the mouse. 



Parasitic Arthropods. 

 - The protozoans and 

 worms are all internal 

 parasites, but among 

 the arthropods are 

 found both internal and external feeders. To the latter 

 group belong most of the mites and ticks of the class 

 Arachnida. The hair mite (Fig. 150) is common in 



FIG. 367. The mite producing sheep scab. 

 Much enlarged. 



