96 PRIMITIVE ANIMALS 



the \ 7 entral aorta, which convey venous, i.e. de- 

 oxygenated, blood from the heart. The blood is 

 oxygenated in the capillaries of the gills, and is then 

 brought back by a series of efferent branchial 

 arteries which join to form the dorsal aorta, and so 

 the arterial or oxygenated blood is distributed to the 

 various organs of the body. 



The ordinary fish is entirely adapted to breathing 

 in the water, so that exposure to the air in a very 

 short time leads to death by asphyxiation. 



But just as some fishes can progress upon the dry 

 land, so there are fishes which can withstand suffo- 

 cation during a prolonged absence from their natural 

 element. The fresh-water Siluroid fishes, which occur 

 in the tropics, Clarias and Saccobranchus, have 

 special chambers leading from their gill-pouches 

 which are adapted for aerial breathing, and it seems 

 that these fishes, even when living in the water, take 

 periodic gulps of air without which they may become 

 asphyxiated in the stagnant muddy pools in which 

 they dwell. Many examples might be adduced of 

 fish having their gill chambers modified for aerial 

 respiration, but of more profound interest are those 

 fishes which make use of a separate organ, the air- 

 bladder, for respiration, since this organ is un- 

 doubtedly the homologue or forerunner of the land 

 Vertebrate's lung. The air-bladder does not occur in 

 the Selachians, but in bony fishes of all sorts it is a 



