204 NATUEAL HISTOEY. 



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 lai-ge 

 dimensions. After destroying the 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. Th& 

 usual length of Boas is from fifteen to thirty feet, but 

 there is a well-authenticated accoimt of the killing of one 

 which measiu'ed sixty-two feet. 



339. There remains to be considered another order of 

 reptiles — the Amphibia (a^^tfitoe, amphihios, 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 

 fishe*?. 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, Avhich 

 is at first a Tadpole, living 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 Ave have al- 

 ready noticed, and therefore are properly classed with 



