280 



REPTILES. 



BATRACHIAN AMPHIBIA. 



The name BatracJdan (from the Greek parpdxos, a frog) is given 

 to those Amphibians that resemble a frog in their structure and 

 general habits. All the Batrachians, when young, undergo a 

 series of transformations or metamorphoses, so that they enter 

 life under an entirely different form from that which they after- 

 wards assume. In their first condition they are usually designated 



Tadpoles. When the young tadpole first leaves the egg it resembles a 

 little fish, and can live only in water. Its head is very large, its belly protu- 

 berant, and its body quite destitute of limbs ; it is provided with a compressed 



tail, which subsequently be- 

 comes elongated and much 

 expanded. Its mouth is a 

 small, scarcely perceptible 

 hole, and its branchiae consist 

 only of a tubercle placed on 

 each side of the hinder part 

 of the head. These appen- 

 dages, however, very soon 

 lengthen and become divided 

 into shreds (Fig. 301 a). The 

 eyes grow perceptible through 

 the skin, and a small trans- 

 verse slit appears under the 

 neck, forming a sort of mem- 

 branous operculum. A little 

 later the branchiae become ramified, and the lips are covered with aminute horny 

 beak, by the aid of which the little animal fixes itself to vegetables that form its 

 chief food ; but this state does not last long. At the end of a few days the bran- 

 chial fringes, appended to each side of the neck, begin to disappear (Fig. 301 b], 

 and respiration is carried on by means of small tufts of blood-vessels, placed 

 along four cartilaginous arches situated under the throat. A membranous 

 tunic, covered by the skin, envelopes these internal branchiae, to which the 

 water arrives by the mouth, and, after having laved these organs, escapes 

 through one or two external slits, the situation of which varies a little in different 

 species. The respiratory apparatus now essentially resembles that of fishes. 

 Some time afterwards the hinder legs of the tadpole show themselves, and are 

 developed little by little (Fig. 301 c}. These attain considerable length before 

 the front legs are perceived beneath the skin, which at a later period they 

 penetrate. About the same time the horny beak falls off, leaving the jaws 

 unencumbered. The tail begins to waste away ; the lungs are developed, and, 

 in proportion as these organs become more exclusively the seat of respiration, 

 the branchiae fade and disappear. Finally, in frogs and toads the tail is 

 altogether lost, the animal assumes the form that it preserves through life, 

 and completely changes its regimen. From being at first herbivorous, it 

 gradually becomes exclusively carnivorous, and all its digestive apparatus is 

 changed accordingly. The period of these changes varies from about four to 

 eight weeks, according to the species ; and it has been ascertained that different 



FIG. 301. TADPOLES. 



