286 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
adjustment to varying hydrostatic pressures, but how far this method is available 
during the changes of level which are encountered in ordinary locomotion is very 
doubtful, and on this point two suggestions may be made : — (1) It may be possible 
that the increase of external pressure which accompanies descent, conditions an 
increased rate of secretion, while a diminution of pressure during ascent involves 
a like acceleration of the absorptive process, with the result that if such vertical 
. movements are not too rapid or sudden, or too extensive, the processes of secretion 
or absoi’ption may keep pace with them, and the Fish retain a plane of equilibrium 
at all levels ; (2) without entirely denying that the former suggestion may be true in 
some subordinate degree, it may yet be conjectured that pressure adjustment by 
such means is more likely to be of advantage to the Fish during slow or gradual 
changes of depth, such as may occur in the course of diurnal, seasonal, or other 
periodic migrations, than in such ordinary and rapid changes of level as are charac- 
teristic of normal locomotion. The slow rate of gaseous secretion and absorption 
renders it difficult to accept unreservedly the first alternative, and this objection 
is further supported by many features in their habits, which tend to prove that most 
Physoclisti have but a comparatively restricted vertical range, in so far as ordinary 
locomotor movements are concerned, as well as by certain facts already quoted.* 
The second suggestion has also this much in its favour — that even a slow rate of 
secretion and absorption would be distinctly advantageous to these Fishes in view 
of the varying pressures to which they are exposed during periods of migration, 
whether the latter are due to variations in temperature and food supply, or to their 
breeding habits. On the whole we incline to the opinion that most Physoclists have 
but a restricted capacity for pressure adjustment during ordinary locomotion, and, 
in this respect, are less favourably situated than the Physostomi. 
Leaving the Physoclists, we may next proceed to discuss in the light of Moreau’s 
researches the mode in which pressure adjustment is effected in the Physostomi. The 
great majority of the wholly or partially freshwater families of this group possess both 
an air-bladder and an open ductus pneumaticus, and to these there is added, in the 
five families of Ostariophyseae, a Weberian mechanism. The relatively few Physo- 
stomi that possess an air-bladder but have no pneumatic duct, are in precisely the same 
position as the majority of the Physoclisti, and whatever capacity for pressure 
adjustment they may possess must be due solely to gaseous secretion and absorption. 
Of the remainder it will be convenient to consider, in the first place, the Ostario- 
physeae, and, secondly, those forms in which the air-bladder is said to possess an open 
ductus pneumaticus, but has no Weberian apparatus. 
From the universal absence of retia mirabilia in all hitherto investigated Ostario- 
physese, it may be legitimately inferred that whatever capacity for gaseous secretion 
and absorption they may possess must be exercised with extreme slowness, and 
therefore as a means of pressure adjustment is of minor importance. On the other 
* See p. 278. 
