The science of life : fully illustrated in tone and line and including many diagrams

THE FIRST GREAT PHYLUM: VERTEBRATES

Fig. 76. The Common Toad grabs, overcomes, and swallows a worm.

in their prime. Amphibia may be recognized by their smooth, moist skins, which lack hair, feathers, or scales. This naked skin is one of the reasons why (with one or two rare and specialized exceptions) an amphibian cannot venture far from moist places ; possessing neither the dry scales of a reptile nor the air-jacket of a bird or mammal he has no protection against water-loss. Sometimes on very hot days one finds mummified frogs lying on roadsfrogs which have ventured into the arid desert of the road and have dried up before they could find their way off it again. _ Two of the three living orders of amphiblans are well known. ‘The first, the frogs and toads, are tailless, and have long powerful hind limbs by means of which they leap or swim. The second, the newts and salamanders, swim by fish-like sideways wriggling of the body, which tapers behind into a long tail; they generally have four limbs, which they use for crawling on land, but in some forms the limbs are very small, or even absent altogether. The third order includes a few rare burrowing forms, having wormlike bodies and lacking both tail and limbs. Nearly all amphibians spend at least a part of their lives in water. They have a fish infancy and an air-breathing adult life. The reader is probably familiar with the life-history of the common frog. Frog-

spawn consists of a mass of clear jelly in which the hundreds of tiny black eggs are embedded ; the embryos are not provided with the apparatus of ensheathing membranes found in the three classes with which we have already dealt. The young frogs or tadpoles have no limbs ; they have large tails with which they propel themselves, and they breathe as a fish does by means of gills. In other characters also they are fish-like. They have, for example, lateral line senseorgans. But as they grow older they develop lungs and also limbs with hands and feet at the ends; they absorb their gills and tails ; they convert themselves from fishes to land vertebrates.

In the great majority of amphibians there is a similar life-history. But some of the salamanders appear reluctant to leave the water ; many of them stay in streams or ponds for their whole lives, swimming up to the surface to breathe, and a few retain their gills until they die, having therefore two alternative ways of getting oxygen. The amphibia are in fact a borderland class. Marshes and water margins are their predestined habitat.

§ 3e Fishes

The fishes are water-living and waterbreathing vertebrates. In the former re-

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