The simplest class is that of the hydroids (Hydroida), the type of which is the fresh-water hydra, so-called because, like the Hydra of ancient myth, when it is cut to pieces each part will grow into a new animal. It lives in ponds and pools of stagnant water, and is so small that a magnifying glass is necessary to study it, especially in the case of the green one of our two common American species—the other is brown. Indeed, similar hydroids of salt water are often taken and dried by unscientific collectors under the impression that they are feathery seaweeds. It is stalklike in shape, has long tentacles which always turn toward the greatest light, influenced like certain plants by heliotropism, and feeds on minute crustaceans and other minute organisms. Sometimes hydras are so abundant as to form a velvety surface in warm pools. The sexes are combined in the same individual, and the embryo forms within the body, then protrudes as a bud, which finally breaks away and after a time sinks, attaches itself at the base to some support, and grows into a perfect hydra. When quiescent or alarmed the tentacles are withdrawn, and the whole animal shrinks into a little lump.
Such is the general natural history of the group; but the oceanic hydroids have developed a vast variety of forms, and, with increased breadth of life, have added many interesting features and habits. Many of them are single, rooted in mud, or upon seaweeds, rocks or shellfish both dead and alive, and look like flowers of lovely tints; and they reproduce by putting forth separate reproductive parts, called "zooids," of various kinds. Others are in colonies that spread by extensions of the base from which arise other hydroids until a bunch of them are growing side by side; but these groups consist of hydroids differentiated into separate functions, for some devote themselves to capturing food which nourishes all, through the common base, while others produce the buds and eggs by which the colony is increased.
| JELLYFISHES |
| (Medusa aurita. Rhizostoma cuvieri. Cyanea capillata.) |
The most remarkable of these processes of reproduction is that which is represented by the jellyfishes so abundant in all seas, and so beautiful either when seen floating along just at the surface of the summer sea, or when at night they glow with phosphorescence like silvery, greenish rockets in the dark waves. Sometimes they occur in enormous "schools"—as we say of fish—all of one kind, filling the water thickly as far as one can see, and now and then in late summer are cast on the beach in long windrows. They range in size from a pinhead to ten or twelve feet in diameter. So big a Cyanea would probably weigh fifty pounds, but after a thorough drying would yield only a few ounces of semisolid matter, 99 per cent of the creature being water absorbed in its spongy tissues. Some are egg-shaped, others like a bell with a long clapper, but the ordinary form is that of an open umbrella, usually fringed about the edge with tentacles, sometimes short and fine, sometimes few and long, again a crowded circle of long snaky appendages. These elastic hanging tentacles are the means by which the medusa (as such a jellyfish is appropriately termed in science) captures its food, which consists not only of the minute things swarming in the plankton, but of other cœlenterates, small crustacea, fishes, anything in fact that it can entangle in its sticky net and sting to death. Every one of the filmy tentacles is thickly studded with microscopic cells (cnidocells) covered by a mere film, and having a spinelike trigger projecting from it. If this trigger is touched, or the film broken, out springs the coiled thread dart which is barbed and carries into the wound it makes a poison that benumbs. Thousands of these microscopic darts may prick the skin of a captive, and paralyze its strength—as it does that of a man who gets caught naked in the trailing net of one of the great northern medusæ. Being thus captured, the prey is drawn up to the mouth, which opens in the center of the under side of the umbrella float.
At intervals around the margin of the umbrella are small organs by which, it is believed, the creature maintains a sense of balance and direction, and perhaps of temperature or light, or both; for many medusæ sink out of sight by day and come to the surface at night; and when the sea is rough they descend to quiet depths. Thus they have the power not only to move ahead by the alternate contraction and dilatation of the disk, but to so alter their specific gravity as to sink or rise at will. They thus show the rudiments of both a muscular and a nervous system.
Very interesting, and often of great beauty, are the free-swimming, colonial, hydroid polyps called siphonophores. On a long stem or string are arranged, at the top, a bulb filled with gas or air, as a float, then a series of swimming bells whose pulsations carry the colony about, beneath which are various polyps and tentaclelike appendages, some to gather food, whose digested products circulate through the whole colony, others performing reproductive functions. The variety of form is considerable; and one of the most peculiar, and the only siphonophore familiar to most persons, is the exquisite Portuguese man-of-war, whose prismatically tinted bulb, as big as one's fist, is commonly met with in the Gulf Stream in the North Atlantic, and often is seen in great flocks in the tropics, bobbing on the surface of the waves in calm weather. Beneath that bulb trails a long tuft of tentacles and zooids, performing various functions, and so foreshadowing the division of labor that in the higher animals is effected by the different limbs and organs.
SEA ANEMONES, CORALS, AND SEA FANS
Sea anemones are simply large polyps of more complicated structure than the hydroid polyps. Instead of a simple, baglike, enteric cavity, the slitlike mouth admits food into a flattened gullet which leads to an enlarged digestive cavity. The gullet does not hang free, but is joined to the outer wall of the body by a series of radiating partitions, between which shorter ones extend from the inner surface of the ectoderm; and below the gullet the stomach wall extends in lobes between these partitions, through which holes permit the nutritive juices to circulate throughout the whole body. The whole upper surface of the polyp is covered by short tentacles arranged in circles. A current of water, induced by waving cilia, is constantly flowing in at one corner of the mouth and out at the other, supplying the animal with oxygen and a certain amount of minute food, and carrying off waste; but the anemones capture by means of their tentacles small fishes, mollusks and everything that can be caught and swallowed. As some anemones exceed a foot in diameter, large and powerful prey may sometimes be taken. It is interesting to note that anemones distinguish very quickly between what is good to eat and what is not. Most of them are sitting near shore on rocks or in tide pools, or are clinging to the larger seaweeds or clustered on the supports of wharves where the waves and tidal currents are continually washing about them, often with much violence, and dashing against them strands of weed or the small wreckage always floating in such a place. None of this is seized, or at least is not swallowed; but whether we are to conclude that this choice is made by intelligence, or only by chemical perception is a matter for study. When harm threatens, or when they crave rest, they withdraw all their gorgeous tentacles, infold them within their mouth, and shrink down into roundish gray lumps that attract neither the eye nor the appetite of any marauder.
