i8 CHILDHOOD OF ANIMALS 



a slit at each side of the neck. It finds its food in the water, de- 

 vouring greedily almost any kind of animal or vegetable matter, with 

 a pair of horny jaws made up of a large number of horny teeth 

 closely set together. So it lives and grows for a few weeks. But 

 soon the limbs begin to bud out (Fig. 8), and the lungs develop, while 

 the tail shrivels, and in an extremely short time a number of internal 

 and external changes take place, and the tadpole suddenly leaves the 

 water and becomes a frog. Such a striking change, associated 

 with a change of habit, is called a metamorphosis, and the young 

 animal, before it has gone through the metamorphosis, is called 

 a larva. The method of development is plainly a very condensed 

 and quickened repetition of the ancestral history, and the larva 

 is equally plainly the modern representative of a remote ancestor. 

 We must not suppose, however, that the larva is the unchanged 



^,„ ^^^^ image of the ancestor. 



*""""' \ -r .- ' - The tadpole, when it is 



^1^ _ ^ not swimming, anchors 



^^-. ^ 'X-, ^J^ itself to water-weeds by 



''*^ ^ '"" an adhesive apparatus, 



-^ 1... ' —"• - a kind of sticky sucker, 



Fig. 8. Advanced Tadpole of a Frog, with „„ 4.Tj„ y^^^^r surface of 



the legs visible. "^^ '■"'^ uiiuei buiidce 01 



the head, just behind the 

 mouth. We have no reason to be sure that this organ, which differs 

 very much in different kinds of tadpoles, is a legacy from the ancestor ; 

 it may equally well be what is called a larval organ, a structure 

 developed for the benefit of the tadpole itself. So also the teeth 

 of the adult frog are true teeth, probably much more like the teeth 

 of the fish ancestor than the peculiar horny jaws of the tadpole. 

 These, too, may be new organs, developed for the benefit of the 

 tadpole. It is probable, too, that the tufts of gills visible from the 

 outside are new organs of the larvse, and that another set of gills, 

 lying deeper in the gill-slits, but not present in all tadpoles, is the 

 true ancestral organ of respiration. Every larva is in this way a 

 composite of organs and structures some of which are ancestral, whilst 

 others are new and developed only for the larva. In some cases, like 

 the tadpoles of frogs, the ancestral element is greater, and we may 

 well believe that the larva is a fairly close copy of the ancestor. In 

 other cases, which I shall describe presently, probably the greater 

 part of the larva is new and gives us no true image of the ancestor. 

 The batrachians which lose their tails, the Anura, or frogs, toads 

 and tree-frogs, show almost every stage between a true meta- 



