204 NATURAL HISTORY. 
of the neck, by which, when the animal is irritated, it is 
made to take the form of a hood. While the Colubers 
are very widely distributed in the earth, the Boas are 
confined to hot climates. The latter are Serpents of enor- 
mous size and great muscular strength; and from their 
power of coiling round their victims and compressing 
them, they are able to overcome animals of very large 
dimensions. After destroying tne life of their victims 
by compression, they proceed to swallow them whole; 
and such is the power of distention in their throats, that 
they can do this with men and even with cattle. The 
usual length of Boas is from fifteen to thirty feet, but 
there is a well-authenticated account of the killing of one 
which measured sixty-two feet. 
339. There remains to be considered another order of 
reptiles —the Amphibia (apud¢ibvoc, amphibios, having a 
double life). They are sometimes, also, called Batrachia. 
These reptiles, including Frogs, Toads, Salamanders, etc., 
are intermediate between the other orders of reptiles and 
fishes. When first born, they are, like fishes, possessed 
of gills, and live wholly in the water. Then a series of 
changes takes place, the animal being at length endowed 
with lungs in place of gills, and fitted to live on land. 
This may be exemplified by reference to the Frog, which 
is at first a Tadpole, livmg in the water, having fringed 
gills and a long tail, with which it swims with considera- 
ble agility. It goes through a succession of changes, in 
which it loses its tail and its gills, and gains four legs 
and a pair of lungs. You will find these changes repre- 
sented in my “ Human Physiology,” page 113. Some of 
the animals of this order do not lose their gills in the 
transformation, but, in their perfect state, have both gills 
and lungs. These, in the strict sense of the term, are 
amphibious, or double lived. 
340. In their perfect or mature state the Amphibia 
are, in most respects, like the reptiles which we have al- 
ready noticed, and therefore are properly classed with 
