ON THE AIR-BLADDERS OF FISH. 99 
In Ceratodus, however, although there is no special vessel 
employed to convey blood to the air-bladder, still we see this 
organ taking on a higher character than observed in Acipenser, 
for instance, as the blood returned is purified and carried direct 
to the heart, which arrangement does not, however, exist in 
Amia and Lepidosteus, both of which fishes evidently use their 
air-bladders for respiratory purposes. 
Lepidosiren is doubtless the highest known form of living fish, 
forming a transitional link between Amphibia and Fishes; to its 
air-bladder venous blood is distributed and arterial taken away 
direct to the ventricle. Consequently we find that in this fish, 
owing to the non-development of gills on the two inferior 
branchial arches, the blood is not arterialised there, but passes 
on to the air-bladder for this purpose; for the blood which 
is carried there is from the returning dorsal portion of the 
branchial vascular arches prior to their union to form the 
aorta. In fact, venous blood is distributed to and arterial carried 
away from this organ direct to the ventricle of the heart. In 
Polypterus we observe the same distribution of vessels, but due 
to the gills being present on the lower branchial arches, the 
blood is oxygenated there, and consequently when taken to the 
air-bladder is not venous, as in Lepidosiren, but arterial. 
That the air-bladder is homologous with the lungs* of 
Batrachians and air-breathing vertebrates appear to be proved, 
as transitional links in the position and functions of this organ 
can be traced from the Lepidosiren and Ceratodus (wherein it is 
distinctly lung-like, and opening on or near the ventral surface of 
the pharynx) to the Polypterus and Acipenser down to the 
Teleostean fishes. 
In the bony or Teleostean fishes, we find the air-bladder in 
adults in two distinct forms. I have already observed that in the 
embryo it originates as a tube or offshoot from the alimentary 
canal. This tube may become entirely obliterated, when such 
forms are termed Physoclisti; or else the canal may continue 
pervious throughout life in the form of a pneumatic tube or 
safety-valve, connecting the air-bladder with some portion of the 
stomach or upper part of the alimeutary canal, in which case they 
are termed Physostomi. This pneumatic tube may or may not 
* « The air-sae is supplied with branches from the pneumogastric nerve, like the 
lungs of higher vertebrata.” Grant, ‘Cumparative Anatomy,’ p. 546, 
