PAKASITES. 93 



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

 worm has also its favourite place, and if it has not the 

 chance of getting there, in order to undergo 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 fosses, the kidneys, in a word, 

 all the organs which are in direct 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 which lodge there, except during the act of 

 migration. 



In Egypt, Dr. Bilharz observed a distome in the 

 blood of a man (Distoma haematobium) ,- 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 papulosa); 

 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. 



