From the point of view of the biologist, perhaps the most interesting feature of this stage of tadpolehood is the almost entire correspondence of the structure with that of a fish, although the adult frog is not in any sense a fish. This curious state of things is explained by supposing that frogs have descended from fish-like ancestors, and that every frog, in the course of its development, is under the necessity of repeating, in a more or less modified manner, the chief stages of its ancestral history. As Marshall happily expressed it,[27] a frog during its development climbs up its own genealogical tree.
The metamorphosis.—Just as the external gills are replaced by internal gills, so these, in their turn, are replaced by lungs, and advanced tadpoles frequently come to the surface of the water to breathe air. Limbs have now grown out from the sides of the body, and the webbed hind feet considerably assist the tail in swimming. Changes take place in nearly all the internal organs, fitting the animal for its life on land; and these changes are so extensive that there is necessarily a short period when the creature is neither tadpole nor frog, and is incapable of feeding. The tail, however, which would be useless to the terrestrial, leaping frog, is gradually absorbed, and forms a store of nutriment during the transformation.
The gills shrivel up, and the slits close; the outer layer of the skin (including the horny jaws) is thrown off, the hind limbs lengthen, and the animal leaves the water—a frog.
EXERCISES ON CHAPTER XVIII.
1. Make observations upon toads. In what respects do they differ in appearance from frogs? How do they move about?
2. Are the skins of toads dry or moist? In what situations have you found toads? How do they protect themselves from the heat of the summer sun?
3. At what time of the year, and in what places, do toads lay their eggs? Compare the voice with that of a frog. Do toads inflate any part of the body when they sing? Compare the vocal sacs with those of frogs.
4. Look for toad-spawn in the spring. It forms long, gelatinous ropes in which the eggs are embedded. How large are the eggs? In what respects do they differ from frogs’ eggs?
5. Keep toads’ eggs in an aquarium, and carefully compare all the stages of development with those of the frog.
6. Count how many caterpillars you can persuade a toad to eat “at one sitting.” What would be one result of the extermination of toads and frogs?