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fresh-water Cyprinide, Characinide, and Siluride, all of which 
are physostomous, while that having the lowest organization, 
its opercular pieces being incomplete, is the Siluride, and 
which alone has marine representatives. Whether further 
investigations will confirm this time alone must show, but if 
such should prove to be the rule, it would seem that this family 
forms a connecting link between such as are marine with those 
which exclusively belong to the fresh-water. 
Among the marine Physostomous forms all that have been 
examined, I believe, and in which communications exist between 
the air-bladder and internal ear, have such by means of ccecal 
prolongations from the air-bladder, and not by a chain of 
auditory ossicles, which appear to be absent in marine fishes. 
This leads one to enquire whether there are any fresh-water 
fishes that have this connection, as observed in sea forms. The 
perch has no auditory ossicles, and I should think his origin 
may be given as marine; and the same conclusion may be come 
to of the trout and anadromous salmon. I have frequently 
thought over which theory is most probable among the 
Salmonidz, whether they were originally marine forms, some 
of which have taken to a. fresh-water life? or whether they 
were originally fresh-water, and some had chosen the sea to 
live in, but returned to breed in their original homes? The 
great number of cecal appendages, which are most developed 
in the marine forms, has always led me to consider that these 
fishes were originally inhabitants of the ocean; and now that I 
find the connecting link between the air-bladder and internal 
ear is not formed by a chain of auditory ossicles, still further 
probability seems to be added to this view. 
In conclusion, I think it may be affirmed that the air-bladder 
in fishes is the homologue of the lungs of the superior verte- 
brate forms; that in some of the higher sub-classes it serves as 
a lung, depurating the blood; that in the majority of true or 
Teleostean fishes it is employed for one or both of the following 
purposes,—as a float, enabling its owner, by compressing or 
dilating it, to sink or rise to any desired level in the water; or, 
secondly, that it assists hearing, by communicating with the 
