472 On Progressive Development [Serr. 
theless, the respiratory organ would appear to have made its appear- 
ance in so perfect a form rather suddenly on the stage, if we were not 
able to trace it progress towards perfection from fishes themselves 
through other members of the Batrachian tribe, up to the point where 
we have seen it completely formed, and capable of exercising all its 
functions in the adult frog. For this purpose we must return to our 
examination of the class of fishes. 
All fish, with the exception of the genus pleuronectes, are furnished 
with an air-bladder, for the most part entirely isolated from any 
communication with the atmosphere, and inflated with an aeriform 
fluid, secreted sometimes by the internal walls of the airsac itself, 
sometimes by a distinct glandular organ attached to it. The air con- 
tained in this bag is found to vary with the habitude of the animal, the 
quantity of oxygen being increased in proportion to the depth of 
water which it inhabits. 
This air-bag, which is totally imperforate in the least perfect osseous 
fishes, is found to*communicate with the external atmosphere in the 
most perfect osseous, and in the cartilaginous, fishes; in the carp it 
opens by a long canal into the stomach, in the sun fish and in the stur- 
geon it communicates with the cesophagus. 
In the proteus anguinus and the siren lacertina, animals belonging 
to that division of the Batrachia called perennibranchia, from the 
circumstances of their retaining their branchie and their aquatic mode 
of life during the whole term of their existence—we find two air sacs, 
very similar in appearance to the air-bladders of fishes, each communi- 
cating by a narrow membranous tube with the pharynx. Upon these 
sacs a minute branch sent off previously to the origin of the branchial 
arteries, is seen to ramify, but the influence which can be exerted on 
the circulation by this means is too slight to be taken into considera- 
tion. 
Advancing one step higher in the scale, we come to animals 
which atacertain period of their life lose the organs of aquatic 
respiration, and breathe atmospheric air only by means of lungs—in 
short, undergo the metamorphosis we have been considering in the 
larva of the frog. 
This change is first observed in the tritons or salamanders, 
belonging to that family of Batrachia which from the circumstances 
indicated has derived the epithet caducibranchie. In these animals 
the lungs still retain the form of simple sacs, in the upper and back 
part of which a cellular structure and more complex ramification of 
the pulmonary vassels begins to appear—a structure which is at length 
