THE ANIMALS OP LAKES AND RIVERS 195 



blotches. The larvae, known as axolotls, have large 

 external giUs, slender hmbs, and a tail fringed with 

 a swimming membrane. Now, at times, under natural 

 conditions, but for causes not clearly understood, these 

 larvae may become sexually mature and lay eggs, while 

 remaining in the water, and while retaining larval 

 characters. That is, of the Amblystoma eggs laid in 

 a lake some may give rise to the salamander-like adult, 

 while others may breed while retaining tadpole charac- 

 ters. As a much rarer phenomenon this may occur 

 in the common newts, which sometimes breed while 

 retaining the characters of the tadpole. 



In a more advanced form the same condition occurs 

 in certain other amphibia, which, as it were, never grow 

 up, but remain permanently in the tadpole stage. In 

 the lakes and swamps of the eastern part of the Missis- 

 sippi basin, and in some of the Canadian lakes, there 

 occurs an amphibian of about one foot in length, with 

 three pairs of external gills and a fringed tail, called 

 Necturus maculatus. This animal uses both its lungs 

 and its gills for breathing purposes, and retains its gills 

 throughout life. The same statements may be made of 

 Siren lacertina, the mud-eel of parts of the United 

 States. These are examples of the reacquisition 

 of the exclusively aquatic habit by animals whose 

 ancestors had become adapted for life both on land and 

 in water. Amphibians, it may be noted, have probably 

 been derived from fishes of the ' ganoid ' type^-that is, 

 originated from fresh-water forms (see p. 202). 



This brief account of the air-breathing vertebrates 

 found in lakes and rivers enables us to draw some 

 general conclusions in regard to these. Obviously we 

 can classify them under three headings : (1) The 

 amphibians, which possess gills at some stage in their 



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