ON THE ANATOMY OF FISHES. 
287 
hand, Moreau’s researches clearly demonstrate that those Ostariophyseae with which 
he experimented, possessed the compensating advantage of being able to substitute 
for absorption the mechanical liberation of gas through the ductus pneumaticus. 
This conclusion is also supported by many observations which go to prove that while 
free in the water and under normal conditions, many Cyprinidae eject gas through 
the mouth on rising to the surface. Hence it follows that such Physostomi possess a 
distinct advantage over all Physoclisti in the fact that they can, during ascent, more 
rapidly adjust the volume of gas in the air-bladder to the requirements of a 
diminishing pressure by liberating the necessary quantity of gas than by relying 
solely on the slow process of absorption, and consequently will retain perfect freedom 
of movement at all points in the course of an upward career, even at the most 
superficial level. But while it must be admitted that in movements of ascent the 
Ostariophyseae possess a distinct advantage over all Physoclisti, it is equally clear 
that in descent the converse must be true, inasmuch as the absence of retia mirabilia 
in the former group will render the process of adjustment to an augmented pressure a 
much slower one than is probably the case in the latter. On the other hand, as 
Moreau points out, a Fish will incur more danger by rising above the plane of least 
effort than by sinking below it for the same distance. It is conceivable that a 
Physoclist in the course of rapid ascent might so far depart from its normal plane of 
equilibrium as to be forcibly carried to the surface of the water, and in that helpless 
condition fall an easy and conspicuous prey to predaceous birds or fishes before it could 
re-adjust by absorption the volume of gas in its air-bladder to the necessities of a 
more superficial level. A similar movement of descent, even if increased pressure so 
reduced the volume of gas in the bladder, and to an equivalent extent increased the 
specific gi*avity of the body, as to remove the Fish from its normal equilibrium, 
would probably involve no special inconvenience, while the gradual secretion of the 
necessary gas would, sooner or later, restore it to a plane of least effort at the deeper 
level. Like the Physoclist, and for the same reason, it is probable that the Ostario- 
physeae incur no serious disadvantage in descent, although in this group the necessary 
readjustment by gaseous secretion takes place still more slowly. And, moreover, this 
apparent disadvantage may be open to some qualification, for it has been experi- 
mentally shown that an increase in the rate of gaseous secretion may be brought 
about not only by augmented pressure, but also by any abstraction of gas that may 
have occurred during a previous ascent. It is, therefore, possible that such conditions 
may lead to a more speedy readjustment to the greater pressure of a deeper level than 
at first sight might be supposed. These considerations seem to us to justify tlie con- 
clusion that, as compared with the Physoclisti, the Ostariophysem possess a greater 
capacity for adapting themselves to rapid and considerable changes of level, more 
particularly in the dii'ection of ascent, and that this is the case is to a large extent 
borne out by what is actually known of their habits and their greater freedom of 
movement at widely different levels. 
