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Dealing with Gravity

Animals who climbed out of the water found that they needed to learn to deal with gravity. This required the use of more energy and the development of stronger muscles, bones, and hearts.

Gravity and Structural Support

In the water, many animals are almost "weightless" because the water supports their bodies. When animals crawled out onto the land they were suddenly seized by gravity. What a surprise that must have been! Tiny, crab-like animals, the ancestors of the insects, kept their small size and light-weight exoskeletons. Gravity was not a major problem for them, and many eventually grew wings and learned to fly.

Larger animals with internal skeletons had to adapt to gravity. This required stronger bones, muscles, hearts, and ligaments. These animals evolved from small ancestors, gradually developing bodies that could deal with gravitational stress.

Exoskeletons and Endoskeletons 

   On earth we see two types of skeletons: exoskeletons (think of exit (outside) to remember which is which) and endoskeletons, in which the body is supported by bones inside the skin and muscles.

crab

 Exoskeletons 

     Exoskeletons are outside the body and encase it like armor: a good example is a crab in its shell. Exoskeletons protect the body. They are light and very strong, and provide attachment places for the muscles inside. They protect the body from dehydration, predators, and excessive sunlight.

     Exoskeletons lead to some difficulties, too. The shell does not grow, so it has to be shed periodically. It is a struggle to get out of an exoskeleton, and the animal is vulnerable while the soft new shell hardens, which may take several hours. Exoskeletons are also difficult to repair if they have been damaged, although crabs and many insects can regrow a leg if they lose one. Exoskeletons are used by crabs, lobsters, insects and spiders. These animals are small.

  Endoskeletons

wading birdEndoskeletons are bones inside the body. They support the body so that it can stand and walk, and some structures, like the skull and rib cage, protect delicate and important organs. Endoskeleton bones provide a storehouse of calcium for the body, and the calcium can be drawn on and used in other ways if necessary. Endoskeleton bones do heal if they get broken, though a doctor's help is often needed to ensure that they fuse together properly.

     Animals with endoskeletons can grow easily because there are no rigid outside boundaries to their bodies. They are vulnerable to wounding from the outside, but repair of the living tissue is usually not a problem. They have nerve endings in their skins that tell them about the environment: whether it is hot or cold, wet or dry, rough or smooth. They feel pain when they are wounded. This information is useful.

     Some bodily tissues are supported by cartilage. This is stiff, rubbery tissue. You have cartilage in your ears and the tip of your nose. Some sea animals, such as the shark, have cartilage instead of bone.


Photograph from a Corel CD-ROM : for viewing only, not for downloading.  
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Copyright © 1999.   Elizabeth Anne Viau and her licensors.  All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at t eviau@earthlink.net