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blood-vesselg.. Polypterus is another fresh-water Ganorp, found 
in South Africa. It possesses a double air-bladder, which opens 
into the pharynx on its ventral surface. The Acipenser or 
Sturgeon is the last of this sub-class to which I propose 
referring. It is provided with an air-bladder, but not similarly 
cellular to those already remarked upon; it does not appear 
to be employed in respiration, but its functions seem to be 
restricted to acting as a float. The opening from the air- 
bladder into the alimentary canal is that of a lower type, being 
on the dorsal surface, and it is not guarded by a glottis. 
Thus far I have alluded to the higher classes of fishes; and 
among the Drenorps we see an air-bladder, having a lung-like 
function, opening on the ventral wall of the pharynx, and 
protected by a glottis. But among the Ganors there is a 
divergence from the Dipnorp organisation. Although Amia 
has a respiratory air-sac, opening into the pharynx by a glottis, 
the communicating orifice is on the upper or dorsal, and not on 
the lower or ventral aspect, as occurring in the higher forms. 
Polypterus it is true opens on the ventral wall, but in Acipenser 
the respiratory function has not been developed, and the pneu- 
matic opening is upon the dorsal wall of the pharynx. 
Air-bladders in fishes, as might be expected in organs 
appearing in the lowest class of vertebrates, are of diverse 
forms and sizes, as well as differently protected. Existing in 
the abdominal cavity, as already described, they have usually 
two coats—an external, fibrous, tough, and glistening, and an 
internal, vascular, and mucous one. Between these two coats 
is often seen (especially in the physoclisti) a red glandular 
body, most frequently in its inferiur region, and compared by 
some anatomists to the thymus. This gland seems to have the 
character of a rete mirabile, consisting of a double plexus of 
arteries and veins. 
It has long been known that the gas contained in the air- 
bladder is a mixture of oxygen, azote and carbonic acid, in 
variable proportions, in accordance with species, and even in 
accordance with individuals. M. Armanp Morzav has proved 
among the fishes in which the air-bladder is closed, (Physoclistz) 
en Ee 
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