2 5 8 AMPHIBIANS. 



metamorphoses, commencing their existence immediately after leaving the egg in 

 a larval condition, during which they breathe the air contained in water by means 

 of gills, while in the adult state they breathe atmospheric air by means of lungs. 

 Varying much in external form, these animals nearly always have the bod}- 

 covered with a soft naked skin ; but in a few instances among existing forms scales 

 are embedded in the skin, and most of the extinct forms had a well-developed 

 armour of scales and bony scutes. In some forms a longitudinal fin is developed 

 down the middle of the back and tail, but this is always soft, and lacks the support- 

 ing spinous bones characterising that appendage in fishes. In passing through a 

 metamorphosis. Amphibians are more like the inferior groups of animals than the 

 higher Vertebrates ; and while in the earlier stages of their existence, during which 

 they breathe by gills, they may be regarded as very closely allied to Fishes, in the 

 adult state they come much nearer to Reptiles. The extinct Labyrinthodonts, 

 which are themselves not very widely removed from fishes, and have the basi- 

 occipital bone ossified, serve to connect other members of the class with the 

 Anomodont and Beaked Reptiles. And it may be mentioned here that while in 

 Mammals the skull has continued to be supported by the two condyles of the 

 Amphibians, in the Reptiles the basioccipital bone has developed an intermediate 

 condyle filling up the gap between the two exoccipital condyles, and thus forms 

 a single tripartite condyle like that of the tortoises. Frequently, as in the 

 crocodiles, the lateral elements have tended more or less completely to disappear, 

 thus leaving a condyle formed almost entirely by the basioccipital. 



As already said, the skin of most existing; Amphibians is soft and 

 Skin. 



naked ; it is invested with a colourless epidermis, which is periodically 



shed entire, while the deeper layer is often coloured with blotches or streaks of 



yellow, red, brown, or black. Other colours, however, such as green and blue, are 



produced by pigment-cells, which generally make their appearance under special 



conditions of warmth and moisture. As a rule, the colour of Amphibians varies to 



a great extent with the nature of their surroundings, as is well exemplified in the 



case of the frog, which chang-es its hue according; to the nature of its habitat : while 



the tree-frogs harmonise with the foliage among which they dwell. It is, however, 



very remarkable that in Costa Rica a certain toad simulates to an extraordinary 



degree the coloration of the snakes— both poisonous and harmless — of the same 



country ; while in North Sumatra Amphibians of various groups are spotted with 



carmine-red. In all Amphibians the skin is furnished with glands secreting a 



more or less milk-like fluid ; these glands being generally distributed all over 



the body, although sometimes they are confined to the sides of the neck behind the 



eyes. In many toads and land-salamanders some of the larger glands appear as 



prominent warts, pierced with large pores. The viscid, milky fluid secreted by 



these glands is exuded during excitement, and is endued with more or less 



poisonous properties, being intended to serve as a means of defence. Although 



some degree of irritation of the skin may be produced by handling some of 



the species in which these poisonous properties are most developed, the stories 



of toads or salamanders spitting venom are, it is almost needless to observe, 



pure fabrications. When introduced into the circulation, batrachian venom acts, 



however, as a powerful poison, influencing the heart and central nervous system : 



