St. Paul daily globe. [volume] (Saint Paul, Minn.) 1884-1896, 1893-01-26
THE ORIGIN OF LUNGS.
Demonstrated by the Air Bladder
in Fish.


The air bladder in fishes is an organ
whose true purpose has long been
classek among the mysteries of animal
organization, says Charles Morris in the
American Naturalist. That the blad-
der was evolved as a swimming organ.
or is of any essential utility for that
purpose. there is the strongest reason
to question. AS it at present exists 1t
is often too small to be of use in
changing the gravity of the fish,
and in many cases it is entirely alb
sent. In others its COMPRISING mus
cles seem incapable of being used to
expand it. Yet in all these cases the
fish seems at no disadvantage vi swim
ming as compared with those that pos
sess large aud efficient bladders. As
all instance, we may cite the mackerel,
when has 10 swimming bladder, and
the great shark tribe, which is also
bIadderiesS.
The bladder varies greatly in size in
closely allied species. A like singular
variation occurs in the case of related
equally marked. Ordinarily it is a
simple sac with smooth interior. In
some instances it has cellular interior.
and in others it is divided by
transverse partitions into from
two to four sections. In other
cases it is divided by longitudinal
partition into two lateral halves. In
some families there is a remarkable de-
velOpment of lateral appendages of va
rying character. In others, the internal
SPECULATION is 50 great that the bladder
resembles the batrachian lung, and evil
IDENTITY does duty in the breathing of air.
The contents of the bladder differ in
different fishes. In fresli-Water fishes Ii
Is nearly pure nitrogen. In marine
fishes, Ell the contrary, oxygen is III eX-
cess. This is particularly the case in
the deep swimmers, III some OF which
the bladder has been found to contain
as much as 81 per cent of oxygen.
These gaseous contents, ii obtained
through secretion by the blood vessels,
are not always so obtained. for i'll many
fishes an arrangement exists by which
air may enter the bladder from without.
This is what is known as the pneumatic
duct, a tubular connection between the
9SOphaguS and the air bladder, not I'M
like that which supplies the lungs Of air
bIeathers. This duet presents the same
remarkable variability observed in other
characteristics Of the air bladder. Its
point of connection with the alimentary
tract varies, being usually in the OesOpha-
gus, but in some fishes as far back as
the atOmach. In the GaDOid order of
fishes the duct is short one, and-al-
ways open. In the PhvSOStOme order
it is longer. and i'll many in
stances closed, being occasionally
reduced to fine tilament. In
the other orders of TeleOsteau
fishes. which embrace the great major
not exist. Whatever function this duct
may have once performed. therefore.
it seems as a rule to have lost its utility.
That its function was an essential one
in the early stages of FISH life is rem
dered evident by the fact that all fishes
which have a bladder at all possess
pneumatic duct in the laryal stage of
growth. =
What, alen, was its primitive func-
ton? In attempting to answer this
question we must first consider the air
bladder in relation to the fish tribe as
a whole. In one principal order
of fshes-the Elasmobranchs-the air
BLADDER does not exist. No shark or
ray possesses this organ. The cOndi-
tons of its occurrence in the 1eleOste-
ans we have already considered. But
in the most ancient existing order OF
fiSheS-the GanOidS-Of which but a few
representatives remain, it exists in an
interesting condition. In every modern
CANDID the air bladder has al
effective pneumatic duct, which
usually opens into the dUrsal
side of the psophaguS. but
in the sub-order Polyterus it opens, like
the windpipe of living Oreathers, into
the ventral side. Finally i'll the small
snb-order of Dipnoi, also survivor
from the remote past. the duet not only
opens ventrally into the Ces0phaguS, but
the air bladder does duty as lung. Elk
ternally it differs in no particular from
ali air bladder, but internally it presents
a cellular structure which nearly ap-
prOaches that of the lung Of the batra-
chianS.
To the current views I oppose the
natural presumption that the duty which
it subserged in the most ancient fishes
was It's primitive function. The facts
of embryoIogy lend strong support to
this hypothesis. Fer the air bladder is
found to arise in manner very similar
to the development of the Jung. The
fact that the pneumatic duct is always
present in the laryal form in fishes that
possess a bladder is equally significant.
All the facts go to show that the intro
duction of air into the body was a
former function of the air bladder. and
that the atrophy of the duct i'll many
cases, and the appearance of the blad-
der in others, are the results of the loss
of this function.