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Page:Popular Science Monthly Volume 35.djvu/684

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THE POPULAR SCIENCE MONTHLY.

better illustrated by certain worm-like animals which live in the stomachs of sharks and other fishes. Being placed where food is ground up fine and all ready for assimilation, there is no need of a mouth, and but little need of a stomach, so both of these organs are lost, and all food is absorbed into the system through the outer walls of the body. Eyes are also lost, and the animal becomes a mere stomach; but, as for that matter, most animals are a mere stomach, with a few necessary organs to assist it. Some true fishes can be classed as parasites, while many use other animals as a means of attachment and protection. Under the dome of the true bell shaped jelly-fish, a species of fish is generally found that is never taken under other conditions. It appears not to be affected by the stinging cells of the animal, but will stay near the mouth while the darts are exerting their deadly powers upon some brother fish, and after this fish is dead will pick up enough for a meal from what the jelly-fish does not eat. What benefit this fish is to the jelly-fish it would be impossible to say, but in such cases some service is usually returned, such, for example, as that of warning the friend in case of danger. This habit of commensalism, or eating at the same table, is seen in other animals, as the oyster crab, pilot-fish, and others. They seem to recognize their friends, and not only do not harm but even protect them. The oyster crab could, if so inclined, devour the oyster without trouble, but it never offers to. Under such conditions certain apparently deadly powers have no effect, and these animals may even be entirely unharmed by digestive fluids. Fish are sometimes found in very odd places. One burrows into the side of a larger fish and stays there, as in a house, catching what food passes by. Another fastens itself on to the sides of a fish by means of a sucker, and, assuming a similar color to that of the larger one, is easily overlooked by its enemies. One of these, the lump-fish, is a very pretty green in color. There are certain fishes that always stay in the surf near shore, being able to remain there without being cast ashore, and never seeking quiet water.

Among the patches of sea-weed which float in the Gulf Stream there are numerous small fishes very prettily colored. One among these has a curious mode of defense, and because of this is called the file-fish. Normally folded down upon its back is a rather long spine. Whenever danger is apprehended, this spine suddenly springs upright, and is held there by a little bone behind it near the base and under the skin. If this bone is touched with a knife, it can be pressed down, and then the spine will fold back; but, unless the bone is removed, the spine will remain rigidly upright. The fish possesses the power of raising or lowering the spine at will. We sometimes see flying-fish jump out of the water and scale along the surface for many feet. Chased by their enemies.