280 
PROFESSORS T. W. BRIDGE AND A. C. HAD DON 
with the vaiying tensions of the gases contained in the alr-hladder. It is clear, 
however, that the Fish is not only exposed to varying hydrostatic pressures according 
to the depth at which it lives, but, at the same time also, to similar variations of 
atmospheric pressure. According to Hasse’s view it is the perception of hydrostatic 
pressure variations with which the Weberian mechanism is more directly concerned. 
The late Dr. Sagemehl (33) also adopted Hasse’s theory, at least so far as to 
regard the Weberian apparatus as constituting a register of pressure variations, but 
with this important modification, that it is not hydrostatic but atmospheric pressure 
which the Fish is thereby enabled to appreciate. Postponing for the present any 
further reference to Hasse’s theory, it becomes necessary to ascertain what measure 
of truth there may be in Sagemehl’s barometrical theory, more especially as it has 
not hitherto been the subject of any critical examination. 
Apart from certain anatomical features in the structure of the air-bladder in the 
Siluridm on which he lays considerable stress, but which are equally reconcilable with 
Hasse’s views, the only special reason advanced by Sagemehl in support of his 
barometrical theory is that most of the Ostariophyseae, and especially the Silurida3, 
live at the bottom of the water, and hence an air-bladder as a hydrostatic organ is 
of very little value to them. We must confess that we fail to see that the argu- 
ments with which he supports his ingenious theory are conclusive or even weighty, 
and in opposition it may be urged : — 
(1.) That the majority of the Ostariophyseae are certainly not “bottom” or 
“ground” Fishes in the same sense as, for example, are the Pleuronectidae, and 
unless they habitually rest on the bottom when not in motion by the exercise of 
their fins like the Flat Fishes, it is clear that an air-bladder as a hydrostatic organ 
must be as useful to them as to other Fishes. Moreover, there are many facts which 
tend to prove that whenever any Ostariophyseae assume a strictly “ground” habit, 
as, for example, in the Loaches {Nemachilus, Cohitis, Botia, &c.) among the 
Cyprinidae, and such forms as Glyptosternum, Exostoma, Euclyptosternum, Amblyceps, 
&c., in the Siluridae, the air-bladder necessarily becomes useless for hydrostatic 
purposes, and invariably undergoes more or less degeneration, just as it does in many 
other Teleostei {e.g., Pleuronectidae) under similar conditions, whereas by far the great 
majority of both families possess well developed and normally constructed bladders. 
(2.) To a Fish at a depth of, say only 6 feet below the surface of the water, a 
variation of atmospheric pressure sufficient to raise or depress a column of mercury in 
a barometer to the extent of half-an-inch will only involve a variation of pressure 
amounting to less than one-tenth of the already existing hydrostatic pressure, and 
even this trifling difference will become relatively smaller as the depth at which the 
Fish lives becomes greater, while the ascent or descent of the Fish in the water to 
the extent of only 7 inches "would certalidy mask any variation of atmospheric pressure 
to the extent indicated. A barometrical variation of even half-an-inch takes place 
but slowly, and consequently could only be appreciated as distinct from hydrostatic 
