Page:Popular Science Monthly Volume 16.djvu/807

 in the water. In that immense aggregation of minute animals, the sponge, the canals ramifying through the mass are lined with cilia, which cause constant currents of water to pass in at the small pores and issue at the large openings. Thus "the sponge represents a kind of subaqueous city, where the people are arranged about the streets and roads in such a manner that each can easily appropriate his food from the water as it passes along." Cilia fringe the gills of the bivalve mollusks, like the oyster and scallop, or the clam, which can burrow in the sand and send up into the water a long tube or siphon.

The great Greenland whale subsists on the small animals which swarm in the Arctic seas. But how shall the enormous beast capture sufficient of those tiny creatures? Its apparatus is as remarkable as it is unique—a huge sieve, made of the fringed edges of hundreds of "whalebone" plates, hanging from the roof of the mouth. Filling its cavern-like mouth with water containing the small animals, these are strained out as the water is expelled.

Solid food in mass requires some means of grasping—true prehension; generally accompanied by the power of dividing or crushing—mastication. A most curious method, and but one step higher than shown in the tapeworm, is exhibited by the microscopic amoeba, found in fresh water. An animal without any permanent appendages whatever, a bit of almost structureless protoplasm, it nevertheless moves without limbs, breathes without gills, seizes food without prehensile organs, and digests without a stomach. All the animal functions are performed by the general mass of the body. Its mode of feeling is as follows: When a nutritious particle comes in contact with the body, the surface at that point begins to depress or fall in, and so continues until the surrounding surfaces meet and unite. In other words, the animal wraps itself around the particle, and the bit of food is enveloped in the albuminous body-mass. The nutritive matter is absorbed, and any undigested or waste matter is expelled by a reverse process. Briefly, the amoeba extemporizes a stomach upon the place and at the time it is needed, and is not troubled with that uneasy organ when it is not needed. The ills of dyspepsia are to the amoeba unknown.

The polyp or sea-anemone has numerous grasping arms called tentacles surrounding the mouth, which is at the top of the stump-shaped body. But the muscular power of the soft, watery animal is not sufficient to hold a lively crab or other struggling prey. To supplement this weakness, it is provided with a most marvelous and deadly apparatus. The surfaces of the tentacles, and frequently of the stomach and body-walls, hold countless minute sacs containing beautifully coiled filaments, which are quickly thrown out like so many poisoned darts to pierce and paralyze the victim. The structure and action of these stinging threads is one of the greatest wonders of nature. The weak jelly-fish uses the same means to overpower its prey, which, enveloped and paralyzed by the hundreds of thread-like tentacles, is drawn