278 
PROFESSORS T. W. BRIDGE AND A. C. HADDOX 
least effort, the Fishes acquire a capacity for the maximum amount of locomotion with 
a minimum of muscular effort.* 
(2.) In its movements of ascent and descent the Fish becomes exposed to augmented 
or diminished pressure, which in each case vai'ies in amount according to the variable 
height of the superimposed column of water, and this leads to an expansion or con- 
traction of the air in the air-bladder, and consequently to an increase or diminution in 
the volume of the Fish itself, and thereby to a corresponding alteration in its specific 
gravity, which may temporarily remove the animal from its normal plane of least effort. 
(3.) The Fish has no power of varying the capacity of its air-bladder by direct 
muscular contraction, and its re-adjustment to a new plane of least effort results from 
a gradual increase or decrease in the amount and volume of the contained air to an 
extent proportional to the new pressure, and due to a corresponding modification of 
the processes concerned in the secretion or absorption of gas into or from the air- 
bladder. Hence, by this apparently automatic method of adjustment the Fish will 
find, sooner or later, and whatever the depth of the water and the amount of external 
hydrostatic and atmospheric pressure, a plane of least effort where it will again 
possess exactly the density of the water, 
(4.) That Johannes Muller’s theory of the displacement of the centre of 
gravity upon a longitudinal axis in the case of Fishes with a two-chambered air- 
bladder has no foundation in fact. 
The conclusions embodied in the preceding sections relate more particulai’ly to the 
Physoclisti — by far the largest group of Teleostean Fishes — but it may be pointed 
out that in a general way they apply also to the Physostomi with, however, the 
qualification that in the great majority of the latter group the mechanical liberation 
of gas through the ductus pneumaticus takes the place of absorption as a means of 
adjustment to reduced pressures. 
Although an air-bladder is unquestionably an advantage to the generality of Teleos- 
tean Fishes, there can be no doubt that its presence is also attended by certain 
disadvantages which to a greater or less extent limit and control the locomotor 
activities of its possessor, and that of these the most important is the restriction 
of freedom of locomotion in the vertical direction, and the consequent limitation 
of ordinary locomotion within more or less well defined limits above or below a 
tolerably constant mean position. As Moreau says, when a Fish is in a plane of 
least effort the possession of an air-bladder is a distinct advantage, but for rapid 
changes of level, as in movements of ascent or descent, the air-bladder may prove a 
considerable disadvantage or even a source of danger. In a Fish, he remarks, which 
is exposed to artificially diminished pressure the air contained in the air-bladder will 
greater than that of the water, and consequently is never in equilibrium, but must constantly use its fins 
to prevent itself from sinking. 
* A Fish in equipoise in the water resembles the philosophical toy known as the “ Carthusian 
Diver,” and the slightest exertion of its fins will readily cause motion in the vertical direction. 
