Tadpoles
Suggestions.—Tadpoles may be studied by placing a number of frog’s eggs in a jar of water, care being taken not to place a large number of eggs in a small amount of water. When they hatch, water plants (e.g. green algæ) should be added for food. The behaviour of frogs may be best studied in a tub of water. A toad in captivity should be given a cool, moist place, and fed well. A piece of meat placed near a toad may attract flies, and the toad may be observed while catching them, but the motion is so swift as to be almost imperceptible. Live flies may be put into a glass jar with a toad. Toads do not move about until twilight, except in cloudy, wet weather. They return to ponds and brooks in spring at the time for laying eggs. This time for both frogs and toads is shown by trilling. All frogs, except tree frogs, remain in or near the water all the year.
Fig. 248.—Metamorphoses of the Frog, numbered in order.
Do eggs hatch and tadpoles grow more rapidly in a jar of water kept in a warm place or in a cold place? In pond water or in drinking water? Can the tadpoles be seen to move in the eggs before hatching? When do the external gills show? (Fig. [248].)
Fig. 249.—Tadpole, from below, showing intestine and internal gills. (Enlarged.)
What parts may be described in a tadpole? What is the shape of the tail? Compare the tadpole with the fish as to (1) general shape, (2) covering, (3) fins, (4) tail, (5) gills.
Do the external gills disappear before or after any rudiments of limbs appear? (6, 7, Fig. [248].) Can you locate the gills after they become internal? (Fig. [249].)
In what state of growth are the legs when the tadpole first goes to the surface to breathe? Which legs appear first? Of what advantage is this? What becomes of the tail? Is the tail entirely gone before the frog first leaves the water? Are tadpoles habitually in motion or at rest?
Is the intestine visible through the skin? (Fig. [249].) Is it straight or coiled? Remembering why some fish have larger intestines than others, and that a cow has a long intestine and a cat a short one, state why a tadpole has a relatively longer intestine than a frog.
Compare the mouth, jaws, eyes, skin, body, and habits of tadpole and frog.