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branchiz on the side of its neck. It is, however, more closely 
related to fishes than Amphibians, especially in its form, its 
being covered with cycloid scales, and a fin surrounding the 
posterior portion of its body; while, although it has no branchios- 
tegous rays, it has free branchiz situated inside a branchial 
cavity, which opens into the pharynx by slits, while externally 
it communicates by a single orifice. 
Ceratodus in many respects, especially externally, is closely 
related to the foregoing, although, as previously observed, its 
opening from the air-bladder into the pharynx is lateral, not 
ventral, as seen in Lepidosiren and the higher vertebrata. But 
the heart now becomes of rather a lower type, having a single 
auricle and ventricle, while its blood is not derived from the 
heart or large vessel springing from that organ, but from 
branches of the abdominal aorta or celiac artery. Still its 
air-sac has the function of a lung, discharging at times arte- 
rialised blood into the atrium of the heart.* Thus transitional 
grades in the position and functions of this organ can be traced 
from Lepidosiren and Ceratodus to Polypterus and Acipenser 
downwards through the whole class of Teleostean fishes. 
This leads us on to see that the air-bladder is homologous 
with the lung in its position and function in some of the 
higher orders ; and if a gradation can be traced, it becomes no 
less clear that this homology (whatever its functions may be) 
exists throughout every variety and condition of air-bladder in 
the piscine tribes. The arteries which supply the air-bladder 
in Teleosteans are offshoots direct from the abdominal aorta, 
celiac artery, or last branchial vein; the blood is returned to 
the portal, hepatic, or great cardiac vein. In the highest class 
of fishes we find this organ differently supplied, as it is not 
only the homologue but likewise the analogue of the lung,t 
* “The air-sac is supplied with branches from the pneumogastric nerve, 
like the lungs of the higher vertebrata.” Grant, Comparative Anatomy, 
p. 546. 
+ QUEKETT, who injected a small portion of the air-bladder of this fish, 
found the arrangement of the vessels was precisely similar to that existing in 
the lungs of reptiles. 
