HOW ANIMALS BREATHE. 



745 



become only mud or even entirely dry. It then travels over scorched 

 and dusty ground in quest of water, and has been kept alive without 

 water for six days. Tropical fresh- water fishes are commonly " able 

 to survive droughts, living in semi-fluid mud or lying in a torpid state 



t 



* ll , 1 



Fig. 4.Siredon pisdforme, the Mexican Axolotl. 



below the hard-baked crust at the bottom of a tank, from which every 

 drop of water has disappeared."* 



The lung of vertebrates is an offset, " diverticulum," of the food- 

 canal, and in some form is possessed by all classes of back-boned 

 animals. In the fishes it is represented by the " swim-bladder," which 

 is mechanical in function, serving to vary the specific gravity of the 

 body. Yet in some species it has also a respiratory function. It is 

 quite wanting in those fishes which, like the skate, grovel on the sea- 

 bottom, and it is relatively large in the flying-fishes. In most adult 



358 i 



Fig. h.Periophthalmvs Koelreuteri, a fish which pursues Onchidium a land mollusk on the sea- 

 shore. The large ventral fins serve for a forward leap. (After Semper.) 



fishes the air-bladder is entirely closed, having no communication with 



any other organ ; and the inclosed gas is obtained from the blood. 



This is largely nitrogen in fresh-water fishes, and oxygen in salt- 



* Gunther's "Introduction to the Study of Fishes," p. 24. 



