HOW FISHES BEEATHE. 
351 
The higher of the two aberrant forms, namely, the Mud- 
fishes of Africa and South America, which once occupied a yet 
more exalted place among the Amphibia, have the choice of two 
methods of respiration, in that they possess a pair of rudi- 
mentary lungs, in addition to their branchial apparatus.* 
These functional lungs, which are, undoubtedly, a modified 
bilobed air-bladder, have a cellular structure, “ the cells having 
the same proportional size and form as in the respiratory part 
of the lung of a serpent,” receive through a pulmonary artery, 
derived from two of the aortic arches which have no gill- 
structure developed upon them, impure blood, and return it 
purified through pulmonary veins, and communicate with the 
gullet by a glottis-like aperture (see Fig. 12), guarded by a 
cartilage. 
“ The peculiar modification of the gills,” observes Professor 
Owen, “ and air-bladder of the Lepidosiren, are precisely those 
which adapt them to the peculiar conditions of their existence.f 
In the inactive state into which they are thrown by their false 
position as terrestrial animals, the circulation, which would 
have been liable to be stopped had all the branchial arteries 
developed gills, as in normal fishes, is carried on through the 
two persistent primitive vascular channels. When the lepi- 
dosiren resumes its true position as a fish, the branchial circu- 
lation is vigorously resumed, a large proportion of arterialised 
blood enters the aorta, and both the nervous and muscular 
systems receive the additional stimulus and support requisite 
for the maintenance of their energetic action.” 
Does not such a form, together with those Amphibia which, 
though possessing perfect lungs, yet retain gills, seem in some 
measure to bridge over the gulf between the Fish and the 
higher Yertebrata, the highest even of which retains in ripe 
years a souvenir, so to speak, of a condition structurally more 
fish-like, in the shape of a persistent — gill-less, it is true 
— branchial cleft ? f 
* For this reason the mud fishes are classed together under the term 
Dipnoi. 
t This fish is accustomed, during the dry seasons, to bury itself in the 
indurated mud of the river-banks, and to remain in a state of torpor until 
the return of the rains. 
X Pseudo-branchia, opercular and spiracular gills have not been noticed 
in detail, since, though interesting from a homological point of view, they 
do not play an important part in the respiratory function. 
For fuller details of the varieties of the air-bladder in fishes the reader is 
referred to an article by the Fev. W. Houghton in the Popular Science Re- 
view for October, 1868, and to Fischer’s Versuch uber die Schwimmblase der 
Fische (Leipzig, 1795). 
Mr. Herbert Spencer, in the second volume (pp. 321 et seq.) of his Princi- 
