CONTROL OF ANIMAL PAEASITES 



267 



Bat 



living young about 0.1 millimeter long. These burrow their way, or 

 are carried by the blood, into the muscles, where they feed actively and 

 grow rapidly. They go to all parts of the body, but gather in greatest 

 numbers in the respiratory muscles, intercostals, and diaphragm. The 

 females live and produce young in the intestine for from five to seven 

 weeks, and the first young begin to reach the muscles in nine days after 

 infection ; hence the disease is pro- 

 longed, and, from the nature of the 

 attack, is extremely painful. After 

 thus feeding in the muscles for an 

 undetermined time the full-grown 

 larva encysts (Fig. 120) and may live 

 for years (thirty-one in man, accord- 

 ing to Braun) or until the flesh is 

 eaten raw by some other animal, 

 when the life cycle is repeated. The 

 rat is continually eating its fellows, 

 and since this is the most common 

 host of the parasite, infection of these 

 pests is continuous. The pig eats the 

 dead rats, or the cysts in filthy sties 

 get into its food, and so it becomes 

 a common host. Cats, naturally, are 

 often badly infested. Cooking all 

 pork thoroughly is the safeguard of 

 man, but we should always remem- 

 ber the dish of spaghetti and the 

 typhoid epidemic (p. 242), and real- 

 ize that the center of a roast, a pork 

 chop, or a cake of fried sausage may 



be scarcely warmed through when the outside is browned to a crisp. 

 Here is a fine problem in civic biology. In connection with rat 

 extermination, why not examine the diaphragms of all rats and mice 

 killed, and tabulate and plot on a map the results obtained? Then 

 examine diaphragm and intercostal muscles of all hogs slaughtered in 

 the district, and tabulate and map the results. Placing this map over 

 the other, note whether there is .more trichina in the hogs where rats 

 are numerous and badly infested. File all these maps and tabulations 

 in the school library, so that when the rats and mice are exterminated, 

 the next year's class can examine the pork and thus record progress. 



Man 



FIG. 119. Life cycle and host- 

 relations of trichina worm 



