THE ANIMALS OF LAKES AND RIVERS 195 
blotches. The larvae, known as axolotls, have large 
external gills, slender limbs, 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- 
sippl 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 
N 2 
