180 Descriptive Zoology. 



case with, any other fishes. There is also a pulmonary 

 artery and a pulmonary vein, making the circulation much 

 more perfect. In keeping with this, the auricle is partially 

 divided into two compartments. There are only three or 

 four species, one in Australia, one in Africa, and one or 

 two in South America. The Australian form has a single 

 lung; the others have the lungs double. The African 

 lungfish secures protection during the dry season by 

 burying itself in the mud, where it remains in a snugly 

 inclosed cavity. These " mud nests," as they are called, 

 have been dug up and the fish taken alive and uninjured to 

 Europe. Gills are present and permanently kept. 



It is a general belief among naturalists that the Amphibia 

 originated from such lunged fishes as these we have been 

 considering. Why, indeed, should we hesitate to believe 

 that a race of animals has gradually changed from an 

 aquatic to a terrestrial life in long course of time, when we 

 have all seen this change take place in the individual frog 

 in a relatively short time ? Whether they were first led to 

 do this from the drying up of the water or from pollution 

 of the water, making it unfit for respiration, or whether 

 they were forced to adopt a land life from the competition 

 for a living in the water, or from some other causes or 

 conditions, who shall say ? 



SUBCLASSES OF PISCES. 



'Subclass i. Elasmobranchii, sharks and rays. 



f Teleostei, the bony 



_,. Subclass 2. Teleostomi. \ fishes. 



Pisces. | _ . , . t . , 



[ Ganoidei, "ganoids. 



Subclass 3. Dipnoi, the lungfishes. 



