258 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 body 

 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 

 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 changes 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 ; 



