ox THE ANATOMY OF FISHES. 
279 
tend to assume and retain an augmented volume, and so lessen the specific gravity 
of the animal that it rises to the surface and can only rest with jDart of its body 
in the air. A Fisli, Moreau adds, incurs more danger by rising above its plane of 
equilibrium than by sinking below this plane for the same vertical distance. On this 
point 'reference may be made to Semper’s remarks (34, p. 340) on the behaviour of 
the “Kilch” {Coregonus liiemalis) when caught in nets and drawn to the surface.* 
A further proof of Moreau’s conclusions is also afforded by the appearance of certain 
Fishes when forcibly drawn up or carried to the surface from great depths, or even 
when trawled from quite moderate depths, both the air-bladder and body being 
greatly distended, the former not unfrequently ruptured, while there is often a 
prolapsus from the mouth or anus of the contiguous portions of the alimentary canal. 
It is perhaps not unreasonable to suppose that a Fish, while under normal conditions 
and free in the water, is liable to accidents similar in kind if less in degree to those 
caused artificially by experiment whenever it ventured upon rapid vertical movements 
in the direction of ascent, accompanied by a marked reduction of pressure. The 
restriction of the vertical range of the Fish is clearly due to the slowness with which 
the necessary secretion or absorption of gas takes place, and the facts above quoted 
are in complete agreement with Moreau’s experiments, from which it would appear 
that the adjustment of the volume of gas in the air-bladder to varying pressures is a 
comparatively slow process, the length of which, however, varies greatly in different 
Fishes, and under different conditions in the same Fish. In vertical movements, 
whether slow’ or rapid, as long as they are confined within certain limits, the action of 
the fins may prove a suflScient safeguard against the evil results of a too great or a 
too low specific gravity, but in more extensive movements, more particularly of 
ascent, the controlling action of the fins may be utterly inadequate to prevent the 
Fish incurring considerable danger in the way suggested above. Hence, it may be 
concluded that in the generality of Teleostean Fishes, and more particularly in the 
Physoclisti, the possession of an air-bladder tends to greatly restrict freedom of loco- 
motion in the vertical direction. 
From the conclusions established by Moreau and Charbonnel-Salle, it becomes 
obvious that the varying degrees of tension of the gaseous contents of the air-bladder 
due to variations in the height of the superincumbent column of w'ater, constitute an 
important factor in the physiology of locomotion in Fishes, and hence, in the absence 
of any other tenable hypothesis as to its function, there is a strong a piiom pro- 
bability that the object of the Weberian mechani.sm is to acquaint the Fish with the 
varying degrees of tension to which the air-bladder may be subjected. Hasse (18), 
who was the first to oppose the then prevalent idea of the air-bladder and Weberian 
ossicles in the Ostariophysese being accessory to audition, without, however, entirely 
excluding the possibility that such might be the case in some subordinate degree, was 
also the first to suggest that the object of the mechanism was to acquaint the Fish 
* .See also on the same subject R. Owen (30, p. 496). 
