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The Shapes of Animals
Nature Bulletin No. 698   December 15, 1962
Forest Preserve District of Cook County
Seymour Simon, President
David H. Thompson, Senior Naturalist

The shape of an animal tells a great deal about the kind of life it leads. Unlike common plants which stay rooted to one spot, they are active creatures that move about under their own power. They crawl, walk, run, jump, climb, dig, swim or fly. They hunt food, make homes, produce young, flee from their enemies or fight them. Certain body proportions and types of legs, wings or other features go along with each habit of life. Animals, even when they are at rest, give the impression of being ready to do something or go somewhere.

For example, animals that specialize in jumping, such as the rabbit, frog, flea, grasshopper and kangaroo, have long powerful hind legs. The climbers may have the grasping feet of the opossum and raccoon; the hooked claws of tree squirrels, cats, woodpeckers and many insects; or the suction cups of the tree frog's toes, or the housefly's feet, by which they can walk up a window pane or upside down across a ceiling. The best diggers -- the mole, woodchuck, badger and the underground young of a 17-year cicada -- have short stout forelegs equipped with heavy claws for scooping earth.

However, most animals are not so highly specialized. Usually, each can travel in various ways and perform many different tasks. None is a jack- of-all-trades, able to do a little of everything.

Animals have a functional beauty all their own, and we describe it with such words as grace, poise, rhythm, smoothness of contour, and symmetry -- no matter whether they are as large as a 100-ton whale or as small as a microscopic water flea. What flower can thrill us so much as a glimpse of a bounding deer, a hunting fox, a soaring hawk or merely small fish swirling in an aquarium.

Their charm comes from a simple basic design widespread among free- living animal life. They have a head end and a tail end; an upper side and a lower side; a right side and a left side. In the higher animals the organs of sight, hearing, smell and taste are located in the head end. Animals as low as the earthworm also have a head end which leads the way and is sensitive to outside stimuli even though it has no eyes, ears or special sense organs. Animals are usually bilaterally symmetrical, the right side tends to be a mirror image of the left side. This balance makes it easier for them to steer a course and not so apt to go around in circles like a bird with a crippled wing.

Streamlining and speed go together. The ideal torpedo-like shape is seen in the whizzing flight of the chimney swift, the racing greyhound, the migrating salmon that fights its way upstream for hundreds of miles, and the porpoise which can outdistance fast ships. On the other hand, just as the man who runs the 4-minute mile is differently built from the shot put champion or the wrestler, the slow-moving animals are imperfectly streamlined. They may even be lopsided like the creeping snails or, like some of the anchored sponges, be entirely lacking in definite form or symmetry.

Perhaps the strangest of all transformations in shape is found among the marine flatfishes which include the flounders, halibuts, turbots, soles and others. These start life like any other young fish, swimming along in the conventional upright position with one eye on each side of the head. At an early age they begin to lean over more and more to one side or the other. At the same time the eye on the lower side begins to migrate across the top of the head, or even through the head, to the upper side. Some species lie on their left side, others on the right, and a few don't seem to care which side is up.

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