246 ANIMAL PARASITES AND MESSMATES. 



even five millions found in a similar quantity of human 

 flesh. 



The Trichina sjyiralis produces about a hundred young 

 worms at the end of a week (viviparous); and a pig 

 which had swallowed a pound of flesh (5,000,000 tri- 

 chinae) might contain after some days 250 millions, 

 reckoning that only half the worms hatched were 

 females, which is not the case, for there are more females 

 than males. It appears that trichinae can become sexual 

 in all warm-blooded animals, but the number in which 

 they can become encysted is not so great. It appears 

 that they are not encysted in birds. 



In the month of December, 1863, E. Leuckart wrote 

 to me from Giessen ; " The Trichinae are playing a great 

 part at present in Germany (with the exception of 

 Schleswig-Holstein). Two epidemics have made their 

 appearance within a few months, and have produced 

 a veritable panic, so that no person will any longer eat 

 pork. The authorities everywhere are obliged to subject 

 the flesh of these animals to microscopic examination." 



We OAve to Leuckart (1856 and 1857) and to Virchow 

 (1858) the knowledge of the principal facts of the history 

 of these worms. Virchow ascertained by experiment 

 that they become sexual in the alimentary canals at the 

 end of three days ; and these two naturalists discovered, 

 after many researches, that trichinae are neither strongyli 

 nor trichoce^Dhali, but a different kind of nematode, 

 which are hatched in the stomach of those whom they 

 infest, and that their embryos, instead of migrating, 

 establish themselves in the host himself. The embryos 

 of parasites do not usually remain in the animal which 

 gives them lodging ; they are evacuated, as well as the 



