AMPHIBIANS 167 



(American forms) . The body is short and compact, there being in most 

 cases nine vertebrae, and either no ribs or three very short ones; in addi- 

 tion to the vertebrae is the urostyle, a bone which is homologous to the 

 caudal vertebrae and extends backward from the pelvic girdle. The 

 hind legs are usually long and muscular. The head is generally very 

 large; the eyes are large and protruding, and the eyelids well developed; 

 the tympanum is flush with the surface of the skin. Just behind the eye 

 and behind and above the tympanum in the toads is a conspicuous 

 raised glandular area called the parotoid gland (Fig. 81). The general 

 surface of the skin is glandular, and is either smooth or tubercular and 

 warty. The color is often bright, and metachrosis is very general. 



--3 



Fig. 81. — Upper surface of head of Bufo americanus: i, cranial crests; 2, tympanum; 

 3, parotoid glands {after Surface). 



The male, in most forms, has a large vocal sack which opens into the 

 mouth by one or two slits in its floor, by means of which it can produce 

 a call which is often very loud; they can, however, croak without the aid 

 of their vocal sacks, and some species do not have them. The tongue 

 is absent in the tropical family Pipida, but in most of the others it is 

 well developed and attached in front, the hinder end being free so that 

 it can be thrown forward; it is in this manner that the animal captures 

 the insects and other small animals on which it feeds. The eggs are 

 usually laid in the water and the tadpoles which emerge from them have 

 external integumental gills and a swimming tail, but no legs. In the 

 metamorphosis which follows the gills and tail are absorbed, the legs 

 and lungs develop, the head and mouth change their form and rela- 

 tive size, and the animal becomes adult and fitted for a terrestrial life. 



