ORDER INTESTINA. 457 
in one species but rarely extend to many other species. They 
are not only found in the alimentary canal, and the canals 
which conduct thither, such as the hepatic vessels, but even 
in the cellular tissue, and in the parenchyma of the best en- 
veloped viscera, such as the liver and the brain. 
The difficulty of conceiving how they get into such places, 
joined to the observation that they never appear out of living 
bodies, has caused some naturalists to imagine that they en- 
gender spontaneously. It is now, however, quite certain, 
not only that the majority manifestly produce eggs, or living 
young ones, but that many of them have separate sexes, and 
couple like ordinary animals. We may then believe that 
they propagate by germs, sufficiently small to be transmitted 
through the narrowest passages, or that frequently the animals 
in which they live bring the germs into the world with 
them. 
We discover in the intestinal worms, neither trachez nor 
gills, nor any other organ of respiration, and they must re- 
ceive the influence of oxygen through the medium of the 
animals which they inhabit. They present no trace of a true 
circulation, and only vestiges of nerves, so obscure, that 
several naturalists have doubted their existence. 
When these characters are found united in an animal, with 
a form similar to that of this class, we range it here, although 
it does not inhabit the interior of another species. 
Every one knows to what an extent the intestinal worms 
injure the animals in which they are too much multiplied. 
Against those of the alimentary canal many remedies are em- 
ployed, of which the most efficacious appears to be animal oil, 
&c., mixed with oil of turpentine. 
We divide them into two orders, perhaps sufficiently dif- 
ferent in organization to form two classes, if adequate obser- 
vation could affix their limits. 
