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| This starfish is forcing open a mussel before consuming it. photo Brocken Inaglory |
The starfish begins existence as a very small egg either dropped into the sea-water by the female, or held upon her body until hatching time. Out of the egg comes a young one, or larva, so small that it can be seen only with a microscope, and as clear as the water itself. Its body is drawn out into several transparent waving arms and the whole little animal is covered with hair-like cilia which, acting as oars, row in unison to carry it about. Floating and swimming at the surface of the ocean, it eats even smaller beings and grows until it can be seen by the naked eye. Then begin great changes; the arms disappear into the rest of the body, and the star begins to develop. When it is complete, though still no larger than a good-sized pin-head, the starfish, too heavy now to be buoyed up by the water, sinks slowly downward to start life on the sea-bottom. After a year there of hunting, feeding, and growing, it may have three-inch-long arms, and perhaps even longer ones as time goes on.
The skeleton is fashioned in star shape, but in some kinds of starfish it is a star of fifteen to forty points rather than but five. A limy network of bars or plates, it is so joined together that the arms or rays can twist and bend as easily as the petals of a flower blown by the wind. Over most of this scaffolding lies a thin layer of muscle and skin of perhaps red, or blue, or yellow color, while between the bars stand little hollow fingers of skin, the “gills,” through which oxygen can pass to all parts of the body. Scattered over and attached to the skeleton are limy spines of different lengths and often clustered about these are smaller spines armed with snapping jaws. This shaggy, bristling cover many times prevents the starfish from serving as a meal for other animals.
The starfish not only lies in sea-water but is filled with it as well. The sea flows in through a small sieve-like oval plate that lies between a pair of arms on the top surface of the animal. From this entry-way it runs down a short canal into a circle of tubing around the hub of the star, and then into the tubes that travel like spokes of a wheel out to the arms. Attached to these arm tubes are hundreds of soft-skinned, so-called tube feet, which are much like very small medicine droppers filled with water and closed at both ends. By squeezing the bulb of the tube foot, the foot itself is stretched out by the water forced into it; when the bulb is relaxed, the water rushes back to it, the foot shortens and may hold on by suction to whatever it is touching.
These little feet, which dangle through the slit on the under surface of each arm, are of use in many ways to their owner. The starfish can glide easily by means of them over the bumps and gullies of the ocean floor. The feet of several arms stretch their fullest in the direction toward which the animal is moving, then as the feet touch something hard, the body is swung forward over them; the feet next shorten and pivot forward once more for a new step. In getting food, even perhaps in searching it out, the tube feet also have their place, for when a sea-worm, a small crab or another starfish is touched, the tube feet hold and pass it to the mouth, a small hole in the center of the under side of the body. When the prospective meal is protected, as in the oyster, by a heavy closed shell, the common starfish of the European and American coasts balances itself on two or three arms and fastens the tube feet of the others against the two halves of the oyster shell. Slowly but surely it pulls upon them until the oyster lies helplessly open. Then out of the starfish mouth billows the stomach, turned inside out that it may digest the oyster on the half shell. Thousands of dollars’ worth of oysters are disposed of in this way each year by hungry starfish.
The starfish themselves have enemies, mainly in the form of larger fishes; but since they can live in warm, cold shallow, or deep ocean waters, and are able to regrow a whole body if even as little as one arm and a fifth of the central hub is left, they have been able to survive their foes and are today one of the most common of all sea animals. Best
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