XI AIR-BLADDER 311 
in the course of diurnal, seasonal, or other periodic migrations 
than during the rapid changes of level which may take place in 
ordinary vertical locomotion." In the generality of Fishes, and 
more especially in the Physoclisti, it may be concluded that the 
possession of an air-bladder restricts freedom of movement in the 
vertical direction, and confines ordinary locomotion within more 
or less well-defined vertical limits above or below the plane of 
least effort for the time being. As illustrating this point, and as 
a proof of the danger incurred by a too rapid rise in the 
water, the following remarks with reference to the “ Kilch,” a 
small Salmonoid (Coregonus) inhabiting the Lake of Constance, and 
a favourite article of food, may be quoted :*—The Fish “are caught 
in nets, and brought to the surface of the water; they come up 
invariably with the belly much distended, the air in the swimming- 
bladder, being relieved from the pressure of the column of water, 
has expanded greatly and occasioned this unnatural distension, 
which renders the Fish quite incapable of swimming. Under 
these conditions the Fish is naturally unable to live for any length 
of time. But the fishermen of the lake have a very simple 
remedy ; they prick into the air-bladder with a fine needle; the 
air escapes with some force, the distension subsides, and the fishes 
are enabled to live under totally changed conditions as to pressure, 
even in quite shallow water and at the surface, swimming quite as 
freely as their companions, the natives of the surface water. 
Hence the Kilch is confined to a certain depth, because it is not 
capable of accommodating the tension of its swimming-bladder to 
the change of pressure in the column of superincumbent water.” 
It is not improbable that the Physostomi, or at any rate 
most of them, are somewhat more advantageously placed in this 
respect. From the general absence of “red glands” in this group, 
it may be inferred that whatever capacity for gaseous secretion or 
absorption they possess must be exercised with exceptional slow- 
ness, and, therefore, as a means of pressure-adjustment may be 
neglected. On the other hand, they seem to possess the com- 
pensating advantage of being able to substitute for absorption the 
mechanical liberation of gas through the ductus pneumaticus. It 
would seem, therefore, that the Physostomi have a distinct advan- 
tage over the Physoclisti in that during ascent in the water they 
1 Bridge and Haddon, op. cif. p. 286. 
2 Semper, Animal Life, Internat. Sci. Series, London, 1881, p. 321. 
