Main->Readings->4th Grade Readings->Table of Contents->Part 3: Amphibians
Vocabulary |
About 350 million years ago, a kind of lobe-finned fish appeared which could crawl up on the land above the surface of the water. They could not breathe air with their gills, but they had swim bladders which already could hold air. These swim bladders developed into small, weak lungs. Lungs are organs which allow animals to get oxygen from air. The animal takes in air and the oxygen passes into the blood inside the lungs. Carbon dioxide comes out of the blood and the animal breathes out.
Now the dry land was open to vertebrate animals. This was the beginning of amphibians. Amphibians are not completely free from living in the water. They must return to it for many important tasks, most need to live where it is wet or moist, and they are not very good at breathing air. Yet they are better at living on land than fish.
Adult amphibians are cold-blooded, like fish. Unlike fish, they have lungs, and they also have eyelids, ears, and voices, things which did not exist in fish. Their skin is thin and moist, and many amphibians get some of their oxygen through their skins.
There are three groups of amphibians, and each one has a different body type. One group is the caecilians (suh-SILL-yans). They burrow in the ground and have long bodies that are shaped like worms. Many do not have eyes. Another is the salamanders and newts, which have tails. The third group, which is the best known, is the frogs and toads. Frogs and toads do not have tails.
The most unusual thing about amphibians is their life cycle, especially in frogs and toads. A life cycle is all the stages in an animal's life from egg to adult. Adult amphibians must return to the water to lay their eggs, because they reproduce in the same way as fish: the female lays eggs and the male swims over them and fertilizes them. Then the eggs, covered in jelly-like material, are left to hatch in the water.
After that, things are a little different. Especially in frogs and toads, newly hatched amphibians are different from their parents. They have gills. They cannot breathe air, and must get their oxygen from the water.
In frogs and toads, this life stage is called a tadpole. Tadpoles do not have legs when they are first hatched. However, after they have lived for a while, they change. Their legs begin to form, their tails drop off, they develop lungs, and they become adult amphibians. This kind of big change between life stages is called metamorphosis (met-uh-MORE-foe-siss), and the amphibians are the only vertebrates that metamorphose.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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Go on to Part 4: Reptiles
This page last modified August 15, 2002
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