432 AMPHIBIA. 



paring the skull of a reptile with that of an amphibian, it will be found that 

 whereas the former has a single knob-like prominence, or condyle, by means 

 of which it is movably articulated to the first joint of the backbone, in the 

 latter there are two such condyles. The embryo, too, of amphibians has a 

 less complex type of enveloping structures, lacking the membranes respec- 

 tively termed the allantois and the amnion. 



Bearing in mind tliese important points of distinction, the student will 

 readily see the propriety of separating the Amphibia as a class apart from the 

 Reptilia. In the adult condition, the members of the present class show 

 great differences of external form. The frogs and toads, for instance, are 

 tailless, four-limbed creatures, in which the hind-legs exceed the front pair 

 more or less markedly in length. On the other hand, the salamanders and 

 newts, although likewise furnished with two pairs of limbs, have a long tail, 

 and thus present a lizard-like form. In the newts, the taU, and sometimes 

 also the back, is furnished with a longitudinal vertical fin, somewhat like that 

 of a fish, although differing from the latter in the absence of any bony sup- 

 ports. Other forms have the body eel-like, and the limbs — of which the 

 hinder pair may be wanting — small and rudimental ; while in some of these, 

 like the olm, the external tuft-like gills are permanently retained. In yet 

 another group, the whole form is worm-like, without any external vestiges of 

 limbs. 



In spite, however, of this great variation in external form, all the ex- 

 isting members of the class present the common feature of having the head, 

 body, limbs, and tail covered with a, soft skin, although in a few scales are 

 buried beneath its surface. The outer skin, or epidermis, which is from 

 time to time cast in a single piece, is always transparent, and the colour is 

 consequently situated in the deeper layer. Various shades of black, brown, 

 red, and yellow, are the predominant colours of amphibians, and these are 

 very generally arranged in large irregular blotches, or in streaks ; but more 

 rarely blues and greens are present. Peculiar glands, which may be either 

 distributed over the whole surface, or confined to the region of the neck, 

 secrete a milky fluid of a poisonous nature, which is poured out under the in- 

 fluence of rage or excitement. And that this secretion serves as a means of 

 defence, will be evident to all who have watched a dog seize a toad. When 

 this secretion reaches its highest development, the apertures of the glands 

 open in the form of warty protuberances on the skin, as is the case in all 

 toads and many salamanders. There are many peculiarities connected with 

 the anatomy of the skeleton and soft internal parts of the Amphibia, but it 

 will suflioe to mention two. Unlike that of all the higher vertebrates, the 

 tongue, when present, is attached to the front extremity of the lower jaw, so 

 that it is protruded by being turned right over as if on a hinge ; and when in 

 repose its tip is directed down the throat. It is always of a thick and fleshy 

 nature, and thus quite unlike the same organ in snakes and lizards. The 

 second peculiarity is connected with the skeleton, in which the ribs are either 

 completely wanting, or represented merely by small rudiments aflixed to the 

 extremities of the long horizontal transverse processes of tlie joints of the 

 backbone. This lack of functional ribs prevents amphibians from breathing 

 in the same manner as the higher vertebrates, in which the cavity of tlie 

 chest is alternately expanded and contracted by the movements of the ribs ; 

 and they are, in consequence, compelled to take in lair in large gulps by a 

 swallowing action. In the presence of a large, thin, flattened bone, lymg °" 

 the under surface of the hinder part of the skull, and known as the para- 



