976 The American Naturalist. (Desinit 
efficient bladders. As an instance may be cited the mackerel, 
which has no bladder, yet which certainly finds no difficulty 
in rising or sinking. The same may be said of the great 
shark tribe, which is bladderless. All this goes to indicate 
that the air bladder could not have been developed originally 
for such a purpose, since its use as a swim-bladder seems of 
such little value to the fish that it has been frequently suffered 
to degenerate and disappear, and even if such a service were 
essential to the fish tribe it is impossible to conceive that this 
organ could have been of utility in swimming in the earlier 
stages of its development. 
Before considering, however, the question of the original 
purpose of the air-bladder, some description of its present con- 
ditions is necessary. This organ is an internal sac, possessed 
by many, but not by all, fishes, and is situated ordinarily in 
the dorsal region of the body, between the vertebre and the 
intestines, and in front of the kidneys. It lies outside the 
peritoneal sac, a fold of which invests its ventral surface. Its 
relation to the surrounding organs varies. In many cases Ib 
is intimately adherent to the vertebral column and the abdom- 
inal tissues, and is often enclosed in osseous capsules formed by 
the vertebrae. In such cases it may readily be compressed oF 
expanded, and the weight of the fish in relation to the water 
be thus changed. But in other instances it is almost loose 10 
the abdominal cavity, and seemingly incapable of acting 84 — 
gravity organ. ee 
It varies greatly both in size and form. In some fishes b 
is so large as to extend into the tail, while in other instances 
it sends processes into the head ; these having some connection 
with the organ of hearing. In many fishes, on the contrary, a 
it is small, sometimes so minute as to be of no conceivable 
utility. In numerous instances it is entirely absent. ©" 
here we find the highly significant fact that variations of this 
kind occur in closely related species. Thus, as has been 
already said, the mackerel has no air-bladder. Yet one exists 
in Scomber pneumatophorus, a species which in every beet 
respect closely resembles the mackerel. Similarly m the 
genus Polynemus one species (paradiseus) is destitute of an Al 
