organs of respiration in animals. 261 
poses of respiration in the sturgeon, and in the three first 
genera of animals mentioned in the present Paper, enables 
us to carry on a regular series of links, in gradation, from 
fishes in general to the myxine; every change in structure 
arising out of some peculiar habit of life, belonging to the 
animal in which it is met with. 
In fishes, the gills are so formed, that the water forced 
from the mouth out at the gills, is applied to them in the most 
complete manner. 
In the sturgeon, while swimming, respiration is carried on 
in the same manner, but when the sturgeon adheres to any 
substance by the mouth, which it has a power of doing by ex- 
tending its lips, some other mode of respiration is required, 
and it is found that in the act of pushing out the mouth, appa- 
rently by the same means, the gill covers are drawn up so 
as to leave a large channel between them and the gills, 
through which the water is brought into the mouth and re- 
turned through the gills; there is also on the inside of the 
gill cover the same structure as on the side of the opposite 
gill, only to a smaller extent. 
In the lamprey, the mouth is more constantly employed in 
laying hold of its prey and other substances, and therefore 
the respiratory organs are not connected with it, but situated 
near it. 
In the myxine, which feeds upon the internal parts of its 
prey, and buries the head and part of its body in the flesh, the 
openings of the respiratory organs are removed sufficiently 
far from the head to admit of respiration going on, while the 
animal is so employed. 
The respiratory organs in the two last genera mentioned 
