AMPHIBIA. 433 



sphenoid, amphibians differ from the existing forms of the higher vertebrates, 

 and thereby approximate to lishes. In certain extinct reptiles, this bone 

 appears, however, to be well developed, so that its importance in classification 

 is less than has been supposed. 



To describe in detail the developmental history of amphibians would re- 

 quire nearly as much space as is allotted in this volume to the entire class ; 

 and it is possible to give only the merest outline. Premising that all the 

 members of the class lay eggs, and that in certain special cases some of the 

 stages of development are abbreviated, the ordinary life-history of an amphi- 

 bian is as follows. The eggs are generally deposited by the female in fresh 

 water and are usually of small size and enveloped in a large jelly-like mass, 

 among which the yolks are conspicuous as dark balls. When these eggs are 

 ripe for hatching, the larvae burst their investing membranes to make their 

 appearance in the world as free-swimming fish-like creatures, provided with 

 a long laterally compressed tail, but devoid of limbs or external gills. They 

 still retain a portion of the yolk, which serves for a short time as nourish- 

 ment. Soon a couple of pairs of feather-like gills make their appearance on 

 the sides of the neck, and give the creature somewhat the look of bearing 

 two pairs of fins. Although in those forms which permanently retain the 

 tail these external gills persist for a considerable period, or even throughout 

 life, in those of which the adults are tailless they soon disappear, and are 

 replaced by internal gills comparable to those of fishes. These external gills 

 are enclosed in a special gill-chamber, from which the water that has been 

 taken in through the mouth is discharged through a single tube or a pair of 

 tubes ; the aperture of the latter being generally single, and situated either 

 on the left side of the body or on its lower surface. With the sprouting of 

 the external gills, the tail rapidly increases in relative size, and soon forms 

 the greater portion of the creature. About this time small bud-like processes 

 indicate the appearance of the limbs, the hinder pair first showing in the 

 frogs and toads, but the front pair being the earlier in the tailed forms. In 

 the earlier stages the jaws are furnished with horny teeth, but later with a 

 kind of beak composed of the same substance. At this time the tadpole 

 subsists on vegetable substances, and, consequently, has a long and much 

 convoluted intestine. It, of course, breathes the air dissolved in the water 

 in which it lives in the same manner as a fish, and has a two-chambered 

 fish-like heart. In the tadpole stage the large tail has no backbone ; and in 

 the frogs and toads this appendage, after the limbs are well developed, is 

 gradually absorbed, and finally disappears entirely. In the permanently 

 tailed forms, on the other hand, this appendage develops vertebrae in its 

 internal axis, when it becomes similar to the tail of a land reptile. About 

 this time the jaws' lose their horny beak, and develop teeth, and the intes- 

 tine becomes shortened. Equally remarkable changes are at the same time 

 taking place in the respiratory and circulatory organs. From the oesophagus 

 a bag-like projection grows out, which, subsequently, divides into two, and 

 eventually forms lungs, which little by little come into use, until the crea- 

 ture is able to breathe both in air and in water. Finally, the gills wither, 

 and the adult frog or salamander appears as an air-breathing, insectivorous 

 or carnivorous reptile-like creature, which has been developed from the fish- 

 like tadpole. During the development of the lungs the heart has changed 

 from a two-chambered to a three-chambered organ ; so that even in this 

 respect the adult amphibian is indistinguishable from a reptile. In most 

 cases, when all these changes have been completed, the animal emerges from 

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