232 PROCEEDINGS 'OE THE SCIENTIFIC ASSOCIATION. 
deeper than in any other part of the body. A number of 
little tubes forming pores start off from it ; each of these 
little tubes has a bundle of nerves. There is something 
very similar to this in the carp. 
In addition to this peculiarity of red flesh in the lateral 
line of the tunny, one of the most distinguished of the 
mackerels, wq have to consider the non-existence of that 
reservoir for air, known as the swimming bladder, placed 
beneath the spine. The gas in this bladder, whether it be 
nitrogen, or oxygen, is a product of secretion. “The air- 
sac is most developed in species which frequent or feed at 
the surface of the water, and is least developed or wanting 
in those which lie at the bottom, or burrow in mud ; its 
secretion contains a larger proportion of oxygen in the 
powerful predacious fishes of deep seas, and nitrogen pre- 
dominates in the feebler species which frequent shores and 
shallow waters. Being developed, like the lungs of higher 
animals, from the alimentary canal, the air-sac of fishes 
generally communicates with the oesophagus or stomach by 
means of a short trachea or ductus pneumaticus ; in some, 
however, this tracheal communication becomes completely 
obliterated, and the sac remains an isolated, closed ca- 
vity, filled with its gaseous secretion .” — ( Outlines of Compa- 
rative Anatomy, by Bober t Grant, M.D., chap, iv., 5th sec.) 
Cuvier very justly observes that whatever opinion may 
be entertained relative to the use of the air-bladder, it is 
difficult to explain how so considerable an organ has been 
denied to so many fishes as occurs in our researches ; not 
only to those which ordinarily remain quiet at the bottom 
of the water, as rays and flat-fishes, but to many others 
that apparently yield to none in the rapidity or facility of 
