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Movement





1. It is true that there are some lowly animals, like Hydra, that spend much of their life fixed firmly to one spot. Others, like Amoeba, progress very simply by the flowing of the protoplasm of the unicellular body.

2. But the vast majority of animals have a complicated mode of progression: they may crawl, fly, swim, or walk. Many can move different parts of their bodies for more than one purpose: flies use their legs for cleaning as well as for locomotion; because of its flexible neck, a bird can turn its head through a wide angle, and preen its feathers with its beak; the higher animals, more especially Man, can carry out complicated series of movement with the fore-limbs.

3. All such movements depend upon the harmonious co-ordination of three factors: there must be muscles on whose contraction and expansion all movement depends; there must be something firm to which the muscle is attached, so that it gets a "pull"; in connection with every muscle there must be nerves which initiate, regulate, and control all muscular action.

4. The necessary support is provided by a skeleton This is a firm structure to which muscles are attached which determines both the shape and movements of the body; it also protects the more delicate bodily organs from injury.

5. In many of the lower animals - in Wood-lice, Beetles, Lobsters, and so on - the supporting framework is an exoskeleton (Greek, exo, outside). Fishes have an exoskeleton of scales, but also an endoskeleton of cartilage or bone.

6. In all Mammals the skeleton, which is composed of bone, is internal and is highly developed. This bony framework must not be rigid, for that would reduce the power of movement to a minimum. To give flexibility the bones of the body move one upon another forming joints, of which there are more than 200 in the body of Man.

Questions:

1. How do some lowly animals, like Hydra, spend much of their life?

2. What mode of progression do the vast majority of animals have?

3. Who can carry out complicated series of movements with fore-limbs?

4. What do all movements depend upon?

5. What is a skeleton?

6. What does a skeleton protect?

7. Is the skeleton of all Mammals highly developed?

8. Why must not the skeleton be rigid?

 







Date: 2015-12-13; view: 513; Нарушение авторских прав



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