235 
thus in Lepidosiren venous blood is distributed to the organ and 
arterial conveyed away, the two efferent veins having coalesced, 
pierce the large post-caval, then pass forwards through the 
sinus and auricle, and thus discharge the blood into the 
ventricle. Consequently we find that in this organ there are 
two distinct modes of sanguification, in the lower division 
arterial blood goes to it and venous is returned from it; 
whereas in the higher forms venous is carried to it, oxy- 
genated at it, and returned as arterial blood into the heart. 
Polypterus, as already remarked, closely resembles Lepidosiren, 
in both, as observed by Owen, the arteries of the air-bladders 
are derived from the returning dorsal portions of the branchial 
vascular arches before their union to join the aorta. In the 
Polypterus the artery of each air-sac is formed by the union of 
the efferent vessels of the last gill; the blood is, therefore, 
arterialised before entering the artery of the air-sac. In the 
Lepidosiren (by reason of the non-development of the gills on 
the two lowest of the branchial arches) the blood transmitted 
to the air-sac is venous. 
If we examine into the condition and uses of the air-bladder 
in the bony or Teleostean fishes we find that they possess 
them in two distinct forms. I have already observed that 
in the embryo this organ is found to be an offshoot from the 
alimentary canal. This communication may become entirely 
obliterated, when such forms are termed Physoclisti ; or else 
the canal may remain pervious throughout life, in the form 
of a pneumatic tube, connecting the air-bladder with some 
portion of the alimentary canal, in which case they go under 
the designation of Physostomi. This pneumatic tube may be 
with or without any distinct valve precluding the inlet and 
exit of air. 
In short in the Teleosteans the air-bladder exists in the form 
of a closed sac, Physoclisti, as in the spiny-rayed Acanthoptery- 
gians, the spineless Anacanthini, the tufted-gilled Lophobranchiu, 
and the hard-jawed Plectognathi. While in the remaining orders 
a connecting duct remains pervious, as in the Physostomi, 
- excluding the family Scombresocide. 
82 
