PARASITES. 93 



bones, as well as in the spinal marrow. Each kind of 

 worm has also its favom-ite place, and if it has not the 

 chance of getting there, in order to midergo its changes, 

 it will perish rather than emigrate to a situation which 

 is not peculiar to it. One kind of worm inhabits the 

 digestive passages, some at the entrance, others at the 

 place of exit ; another occupies the fossae of the nose ; 

 a third the liver, or the kidneys. 



We may even divide parasites into two great cate- 

 gories, according to the organs which they choose : 

 those which inhabit a temporary host, almost always 

 instal themselves in a closed organ — in the muscles, the 

 heart, or the lobes of the brain ; those, on the contrary, 

 which have arrived at their destination, and which, 

 unlike the preceding, have a family, occupy the 

 stomach with its dependencies, the digestive passages, 

 the lungs, the nasal fossae, the kidneys, in a word, 

 all the organs which are in dkect communication with 

 the exterior, in order to leave a place of issue for their 

 progeny. The young ones are never enclosed. Even 

 the blood is not free from these animals, but there 

 are few w^hich lodge there, except during the act of 

 migration. 



In Egypt, Dr. Bilharz observed a distome in the 

 blood of a man (Distoma hoeviatobium) ; the Strongylus 

 of the horse has been long known, which causes serious 

 injuries in its vessels {Strongylus armatus) ; as also the 

 strongylus of the dolphin and of the porpoise {Strongylus 

 inflexus), and the filaria of the dog {Filaria papillosa) ,- 

 and some are also found in the blood of many birds, 

 of reptiles, batrachians, and fishes; so that there is no 

 class of vertebrates which escapes. 



