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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; 
