Imágenes de páginas

Early during its transformation the tadpole loses its gills, these being replaced by lungs. At this time the young animal may be seen coming to the surface of the water after a bubble of air. Changes in the diet of the animal also take place at this stage of metamorphosis; the long coiled intestine is transformed into a


Stages in the life of tadpoles of the green frog. The two large tadpoles are in their second summer. Photographed by Overton,

much shorter one in the adult. The animal, now insectivorous in its diet, becomes provided with tiny teeth and a mobile tongue, instead of keeping the horny jaws used in scraping off alge. After the tail has been completely absorbed and the legs have become full grown, there is no further structural change and the metamorphosis is said to be complete.

The Common Toad.

One of the nearest of the allies of the frog is the common toad. The eggs, like those of the frog, are deposited in fresh-water ponds, especially small pools. The egglaying season is later than that of the frog. The eggs are laid in strings, as many as eleven thousand eggs having been laid by a single toad.

Field Work. The egg-laying season in New York state is early May. At this time procure a female that has not laid her eggs and place her in an aquarium. If undisturbed, she may lay her eggs in captivity. Compare the bulk of the eggs after they are laid with the size of the toad that laid them. This apparent discrepancy is caused by the swelling of the gelatinous substance around them. If possible, count the number of eggs laid by one female.1

Toad tadpoles may be distinguished from those of the frog, as they are darker in color, and have a more slender tail and a relatively larger body than those of the frog. The metamorphosis occupies only about two months at the temperature of New York. During the warm weather the tail is absorbed with wonderful rapidity, and the change from a tadpole with no legs to that of the small toad living on land is often accomplished in a few hours. This has given rise to the story that it has rained toads in a given locality, because during the night thousands of young toads have changed habitat from the water to the land.

The toad is of great economic importance to man because of its diet. No less than eighty-three species of insects, mostly injurious, have been proved to enter into the dietary. A toad has been observed to snap up one hundred and twenty-eight flies in half an hour. Thus at a low estimate it could easily destroy one hundred insects during a day and do an immense service to the garden during the summer. It has been estimated by Kirkland that a single toad may, on account of the cutworms which it kills, be worth $19.88 each season it lives. Toads also feed upon slugs and other herbivorous animals.

Other Amphibians. The tree frogs (called tree toads) are familiar to us in the early spring as the peepers of the swamps. They are among the earliest of the frogs to lay their eggs. During

1 See Hodge, Nature Study and Life.

2 (See Kirkland, Habits, Food and Economic Importance of the American Toad.) Bul. 46, Hatch Experiment Station, Amherst, Mass.

adult life they spend most of their time on the trunks of trees, where they receive immunity from attack because of their color markings. The feet of the tree toad are modified for climbing by having little disks on the ends of the toes, by means of which it is

[merged small][merged small][merged small][graphic]

passing through the stages of the segmenting egg, with few exceptions they breathe by means of external gills; later they may develop lungs. A few never have lungs, but breathe through the moist skin.

Newt. From photograph loaned by the American Museum of Natural History.

Still other amphibians are the mud puppies, sirens or mud eels, and the axolotl. All of the above animals differ from the reptiles in having a smooth skin with no scales, and in passing the early stage of their existence in the water.


ORDER I. Urodela. Amphibia having usually poorly developed appendages. Tail persistent through life. Examples, mud puppy, newt, salamander. ORDER II. Anura. Tailless amphibia, which undergo a metamorphosis breathe by gills in larval, by lungs in adult, state. Examples, toad and frog.



Davison, Practical Zoology, pages 199-211. American Book Company.
Herrick, Text-book in General Zoology, Chap. XX. American Book Company.
Nature Study Leaflets, XVI, XVII. N.Y. Department of Agriculture.

Hunter and Valentine, Laboratory Manual of Biology, pages 170-177. Henry Holt and Company.

Jordan, Kellogg, and Heath. Animal Studies. D. Appleton and Company.


Holmes, The Biology of the Frog. The Macmillan Company.

Parker and Haswell, Text-book of Zoology. The Macmillan Company.


[ocr errors]

The Spotted or Mud Turtle (Chelopus guttatus). For a classroom exercise use living turtles. The body is flattened, and is covered on the dorsal and ventral sides by a bony framework. This covering is composed of plates cemented to the true bone underneath, the whole forming one horny cover. What is the general arrangement of these plates? The dorsal covering is known as the carapace, the ventral one the plastron, the connection between them the bridge. Allow the animal to remain quiet for a moment, then touch the head suddenly. What is one function of the shell? In the box turtle this adaptation is made more evident by a hinge in the plastron which fits over the head and legs after they are withdrawn into the shell.

Adaptations. Place a lively turtle on its back. How does it attempt to regain equilibrium? Notice the long neck. The long neck and powerful horny jaws are of great use to the animal in food getting. Allow the turtle to crawl on the table. Then place it in a dish of water. How are the legs adapted to movement in the water? How is the foot adapted for other purposes?

Turtles are very strong for their size. The stout legs carry the animal slowly on land, and in the water, being slightly webbed, they are of service in swimming. The strong claws are used for digging especially at egg-laying season, for some forms of turtles dig large holes in sandy beaches in which the eggs are deposited.

Watch a turtle feeding. Notice that the claws are used. How? The absence of teeth makes it necessary for the turtle to tear the food with the aid of the strong claws.

Western painted turtle.

The sense of hearing in the turtles is not keen. The tympanic membrane can be seen just behind the eyes on the sides of the head. Can you determine by experiment anything regarding keenness of vision in the turtle? Is the turtle protectively colored? Describe any evidences you may see. Notice that the yellow, ventral side would harmonize the general coloring, looking through the water toward the surface of the pond. The yellow dots on the black background look in the water much like small stones or sand grains.

Draw the turtle, natural size, from the dorsal side and label all the parts you know.

The Turtles. The turtles form a large and interesting group of animals. They are mostly aquatic in habit. Some exceptions are found, however, as in the case of the box tortoise (Cistudo Carolina) and the giant tortoise of the Galapagos Islands. This latter animal attains a weight of three hundred pounds or more and may be over four feet in length and almost three feet in

[ocr errors]
« AnteriorContinuar »