Pisces. 179 



be noted that there are but few living ganoids ; these are 

 the survivors of a host that existed in earlier geologic 

 periods. Many of these fossil forms possess a heavy 

 armor, of which the gar shows a sample. It is also 

 noteworthy that North America has a majority of the 

 survivors. Most of them have heterocercal tails. 



The most valuable feature for our consideration is the 

 air bladder, which is present in all, and in all is connected 

 with the gullet by a persistent open duct This duct opens 

 on the dorsal side of the gullet except in one instance. 

 Further, the air bladder in most has an unusual supply of 

 blood, so that it serves to a considerable extent as a lung. 

 Both the mudfish and the gar pike often come to the sur- 

 face and emit bubbles and take in a fresh supply of air, 



FIG. 115. LUNGFISH. 

 After Boas. From Kingsley's Zoology. 



very much as do the mud puppy and the tadpole after the 

 lungs begin to develop. These two fishes are very tena- 

 cious of life when removed from the water, undoubtedly 

 because of the ability of the air bladder to act as a lung. 

 The ganoids thus foreshadow the lungfishes, as, in turn, 

 the lungfishes anticipate the Amphibia. 



THE LUNGFISHES. 



In the Jungfishes the development of the air bladder as 

 a lung is much more complete than in any of the ganoids. 

 The nostrils open into the mouth cavity, which is not the 



