IDS NATURE STUDIES. 



in many fishes this tube disappears, leaving the air- 

 bladder a closed sac (as in the cod) ; or the duct may 

 persist, and place the sound in communication with 

 the digestive tract, as in the sturgeon or herring. 

 Again, the air-bladder maybe a simple and single sac; 

 -or it may be variously divided, and its interior may be 

 smooth or may be divided into cells. We shall pre- 

 sently see that in the mud-fishes and the " Jeevine," 

 this structure assumes a form and function for which 

 its variations in common fishes in some measure prepare 

 us. Turning now to the last-named fishes (Lepidosiren 

 and Ceratodus), we discover that their fish-characters 

 exist on the very surface of things. Their blood is 

 cold; their bodies are scaly; they have fins and fin- 

 rays ; and above all, they possess gills existing in the 

 sides of the neck, and in which, so long as they swim 

 in the water, their blood is purified. But here the 

 fish-characters end. Another aspect of the mud- 

 fishes and the barramunda reveals characters which 

 startle us as being not those of fishes, but those of 

 frogs ; and frogs, toads, and newts form, as every one 

 knows, the second higher class of vertebrates, that of 

 the Amphibia. 



Firstly, then, the Lepidosiren possesses a heart, 

 which is not that of a fish, but modelled 011 the type 

 of the frog or reptile heart. Instead of being two- 

 chambered, it is three-chambered ; and no other fish 

 save itself possesses such an advance on the ordinary 

 type of fish-heart. But, secondly, their " paired fins," 

 which represent in all fishes the " limbs " of higher 



