THE AIR OR SWIMMINO BLADDER OF FISHES. 
383 
been able to rise to the surface or descend with perfect ease.* 
According to Professor Kner the form and size of the swim- 
bladder,- its position and development *of vesicular glands, not 
only differ in the various genera and species, but, curiously 
enough, they vary in one and the same species of fishes of the 
family Murcenidce. Professor Owen has some interesting re- 
marks on the absence of the air-bladder in the surface-swim- 
ming sharks : — Though the air-bladder serves it also enslaves. 
It opposes, for example, those fishes that possess it in their 
endeavours to turn on one side, and it demands a constant 
action of the balancing fins to prevent that complete upsetting 
of the body which it occasions from the weight of the super- 
imposed vertebral column and muscles when life and action 
are extinct. The sharks require, by the position of their mouth 
and in their common pursuit of living prey, freedom in turning 
and great variety as well as power of locomotion ; if they are 
not aided by a swim-bladder, neither are their muscular ac- 
tions impeded by one ; whilst their swimming organs require 
that degree of development and force which suffices for all the 
evolutions they are called upon to perform.” But it would 
appear that there are exceptions to the general rule of the 
absence of a swim-bladder amongst the Selachians (sharks and 
rays), for, according to the investigations of Micklucho-Maclay, 
certain selachians are possessed of a rudimentary air-bladder : 
is this rudimentary organ the remnant, as it were, of a swim- 
bladder that once existed in a fully developed form in selachians 
of some remote age, or must we regard it as a new and inde- 
pendent growth ? On this question a few remarks will be made 
by-and-by. Undoubtedly, the most interesting point for con- 
sideration is that one which relates to the question whether or 
not the swim-bladder of fishes is the homologue of the lungs of 
air-breathing vertebrates. FunctioDally in all fish, with, I 
believe, the exception of the Protopteri — represented by the 
two species of Lepidosiren, one belonging to the American, the 
other to the African fauna — the air-bladder has no relationship 
with a lung. Its function is generally a mechanical one, of no 
great importance, however, and occasionally, as in the carp, it 
serves to intensify sounds in relation to the sense of hearing. 
There can, however, be no doubt that, under all its diver- 
sities of structure and function, the homology of the swim- 
bladder with the lungs is clearly traceable.” True, there is 
nothing at all in the simple cylindrical, closed air-bladder of a 
perch to remind one of the cellular structure of the lung of an 
* Since the above was written I have found that the experiments are 
recorded in the “ Annales des Sciences Naturelles ” for 1866. The paper 
(Du role de la Vessie natatoire) is by M. Edouard Gomiet, and is very 
interesting-. 
