[Pg 347] 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? [Pg 348] 7. Describe carefully what happens when a frog leaps. Point out the special arrangements which enable a frog to leap safely. (1897) 8. Describe the ordinary process of feeding of a frog, and show how it is assisted by the peculiar structure of the frog’s mouth and tongue. (1895) 9. Explain the special use of the wide gape of the frog. Where are the teeth of the frog situated? (1898) 10. Mention some remarkable features of the mouth of a frog, and try to show that they are adapted to meet special needs. (1901) 11. Describe the process of filling the lungs with air, as observed in the frog and the rabbit. (1896) 12. What changes take place in a tadpole during the first week after hatching? Illustrate your answer by drawings. (1898) 13. Describe the appearance of one of the gills of a very young tadpole as seen by a low magnifying glass. (1901) 14. Describe the appearance, size, and mode of life of a tadpole about a fortnight after hatching. On what does it feed? Describe its mouth carefully. (1897)