Popular Science Monthly/Volume 51/August 1897/A Lilliputian Monster


IN front of a sunny window there stands, on a small bamboo table, an aquarium of very unpretentious appearance and size. It is nothing more than a "globe," such as is used for goldfish. In the bottom are a couple of inches of river sand, with a thin layer of gravel, which was repeatedly washed before it was placed there. Planted in the sand is a plant of water starwort, as well as one of anacharis, while on the surface float a few plants of duckweed; and two or three water snails complete the arrangement. Around the rim of the globe is tied a string on which is threaded a screen of dark-green material, which can be drawn so as to shade the globe or to admit the sunlight at pleasure.

Occasionally I amuse myself by fetching a pailful of water from the stream that runs through the meadows; and as I take up the water I give the grass and water weeds a good shake to get whatever creatures may be hiding there. I have provided two pieces of glass tubing of equal diameter—one about two feet in length, the other only some fourteen inches; and these I have bent nearly double by the heat of a spirit lamp. Thus equipped, I change some of the water. I place a footstool or a pile of books covered with a newspaper on the table, so as to get a surface as high as the top of the globe. On this I place a "Mason jar," borrowed from the kitchen, filled with the water from the stream. By the side of the globe I place another jar, empty. Taking the shorter tube, I place one end in the aquarium and give a quick draw with my lips, slipping the end instantly into the empty jar. As soon as I see that the water is flowing through the siphon I repeat the operation with the full jar, and, as the leg of the siphon is sufficiently long, the water runs from the jar into the bottom of the globe. Thus a current is set up in which the plants wave back and forth, and from the liveliness of

PSM V51 D533 Hydra vulgaris.png
Fig. 1.—Hydra Vulgaris.

the living creatures it is evident that they enjoy the sensation. When I see that the upper jar is nearly empty, I remove the siphon from the lower one, and the replenishing of my "tank" is at an end.

Few can tell how much pleasure is obtained from this apparently trifling "hobby," but it really is intense. Close at hand is the microscope, and on the table, ready for use, lie a piece of glass tubing for a pipette, a zoöphyte trough, a pair of forceps, and an ordinary pocket magnifier. It is easy, when anything is wanted, to take it out, either with the pipette or the forceps, and transfer it to the trough. By holding it up to the light one can readily see whether it is worth while to sit down at the other PSM V51 D534 Fresh water hydra hanging from duckweed.pngFig. 2.—The Fresh-water Hydra hanging from Duckweed in a Pond: 1, the long-armed hydra (Hydra fusca) feeding; a a, small animals caught in it arms; 2, short-armed hydra (Hydra viridis) throwing off young hydra buds, b b. table and use the much more powerful aid of the binocular microscope. My friends, it is true, laugh at me, and I laugh at them. They wonder why I am so devoted to "a glass globe full of water, with a few plants and snails," and I tell them that while they see much to admire in horticulture, agriculture, and a host of other "cultures," I am an enthusiast about hydra-culture. Indeed, in this small and insignificant aquarium I have a flock of freshwater polyps, called "hydras," full of interest, full of wonder. I envy Trembley, who in 1744 published A Memoir on the Fresh-water Polyp, the intense pleasure he felt in unraveling the life history of these creatures. He was investigating the unknown when he studied the strange phenomena connected with them, and was transported with astonishment. I know, from the labors of others, what to expect, and yet I am lost in wonder.

We may be thankful that these animals are as small as they are; for, if they were only a few feet in length, we should have in our water world many a repetition of the devastation said to have been caused by the Lernæan Hydra, whose destruction was one of the gigantic labors of the hero Hercules. As it. is, the longest you can find is only an inch in length. They can, however, be easily seen with the unaided eye, and with the help of a pocket lens can to some extent be studied. In fact, Trembley, the famous observer of them, had nothing better. It is only when we wish to examine minute details that the use of the elaborate microscope is called for. A group of them attached to the rootlets of duckweed or the under side of the leaves or on the stems of plants is a curious sight. A nearer view may often be obtained, for they will attach themselves to the side of the glass to enjoy the light, which they seem to love.

The commonest species of hydras may easily be distinguished by their color, one being usually a reddish brown, while the other is a vivid green. The particular shade of color of the former depends on the nature of the food captured; and it is said that it has been colored blue, red, and white by feeding it with matter stained with these pigments. Other species have been noticed, but they are less common, and some are rarely met with.

A single individual may be thus described: There is the body, like a thick thread, of varying length up to an inch. At the foot the substance is slightly expanded into a small disk, with which the creature fixes itself to some surface; at the top is a series of arms or tentacles, thin, threadlike, and arranged symmetrically around the end of the stem, which is the mouth. These tentacles vary in number, the green hydra having from six to ten, and the brown one from seven to twelve. As the trunk sways this way and that in a slight degree, and the tentacles twist about, the sight is a curious one and well worth while watching. The arms have the power of contracting, and sometimes look like little buds around the mouth; and the trunk itself will also sink down until it seems to be a small mass of jelly.

The trunk is nothing but a stomach. There are no lungs, no liver, no heart, no intestines, no nervous system. All there is consists of two layers of tissue, with a very delicate layer between. The trunk and the tentacles are alike in structure, and are simple hollow tubes. If the latter are examined with a magnifying glass, they are found to be covered with little warts, which are technically called "urticating" or stinging organs. These consist of an oval capsule, the top of which is turned back into itself so as to form a cavity in which there lies a thread coiled up, while round the edge of the cavity are four little darts. In this introverted capsule the thread lies bathed in a poisonous PSM V51 D535 Lasso cells of the hydra and sea anemone.pngFig. 3.—Lasso Cells of the Hydra and Sea Anemone: 1, piece of one of the hydra's arms, showing the cells crowded in it; 2, one of the cells; 3, the same cell after bursting open; 4, lasso cell of an anemone. secretion, and the darts are nothing more or less than poisoned arrows. The opening is provided with a sort of trigger, so that when any substance is pressed against it the capsule is flung outward, the thread is cast round the opposing body, and the poisoned arrows are projected into its substance if it is penetrable.

The hydra lives on minute aquatic creatures, and is exclusively carnivorous. Attached to its moorings, it spreads its arms in every direction with a searching motion; and although the two commonest forms can not explore at a much greater distance than the height of the body, the "fuscous" species—one of the rare forms—has tentacles which can reach some seven or eight inches. As soon as a victim touches the subtle angling line it is seized by it, enveloped in the threads, and struck by the darts. Then "begins a tussle which generally ends in the captive being conveyed to the mouth. Occasionally some strong swimmer may get away, but, unless he is armor-plated, he has but little chance of his life, for a poisoned dart is most probably imbedded in his body. Sometimes a victim is very troublesome, and, in order to get it safely into its mouth, the tentacle itself must also be partially ingulfed, and it remains so until the morsel is quiet, and even until digestion has begun. As the food is drawn in, you can see the body swell, and in some cases become quite pear-shaped. After a while the swelling subsides, and, after all the useful part of the food has been extracted, the rest is ejected at the same place at which it entered. It is most interesting to watch the instinctive motions of this creature, which is totally destitute of what we should call brain.

Hitherto we have spoken only of a single individual, but we must now notice a startling fact. The hydra is multiplied according to the usual law—by eggs, it is true; but also in another way. You can not examine a group in summer time without finding that they "bud." You see the trunk of one bearing a second, perfect in every respect, except that it is connected with its parent, instead of resting on a foreign substance. It has sprouted out from the parent stock, like a sucker from a tree. It may break off after a while and seek an independent resting place, or it may send out a bud from its own stem, which in its turn may do the same, and all may remain attached for some time. While this connection lasts, each member of the compound body forages at his own "will," but the tubes of each connect with the trunk of the next, and so with the parent stomach. Thus they form a colony in which each member helps to maintain every other member by his labor. A sight of such a colony of hydras, the working of which is visible to the naked eye, helps one to understand many other similar forms of animal life—as, for instance, the corals, which form colonies by budding. As many as four generations of hydras have been seen on one stem, so that there is some reason for likening such a community to "a living genealogical tree."

By a number of experiments with which some of us will scarcely sympathize, Trembley and those who have repeated his investigations have brought out many astounding facts about these animals. If you cut one in two across the trunk, the upper part floats off and resumes its voracious habit in a new locality; while the lower portion remains, develops a new set of tentacles, and goes on just the same as if nothing had happened. Nay, you may cut a hydra into five or six pieces, and each will make a separate animal. If one is divided into two vertically, the two halves close up, and again you have two individuals. Trembley succeeded also in turning a hydra inside out, and it was able to catch food and digest it as well as before. The creature, however, insisted on turning itself back again, and this was not what the experimenter wished. He therefore passed a needle through the body, near the mouth, and kept it there.

The method of this Dutch naturalist was very ingenious. Holding the hydra in a little water in the palm of his hand, he induced it to swallow a small worm. He then took a bristle and began to push against the base, working the end of the body upward against the worm, and soon had the animal inverted. Thrusting a needle through the base of the tentacles, he had what he wanted. He says: "I have seen a polyp turned inside out, which has eaten a small worm two days after the operation. I have fed one in this state for more than two years, and it has multiplied in that condition."

Hydras have but low powers of locomotion, but still they can move from place to place. When one wishes to go upon its travels it attaches itself to the surface of its support by a tentacle, and then moves the disk up to the tentacle. In this way it can get over about eight inches in twenty-four hours. It can, however, take a longer journey by attaching itself to the shell of a water snail, and thus travel in a few minutes a greater distance than it could do in a day alone. It can also swim with the disk floating on the surface of the water as if suspended.

Although without eyes and a nervous system, the green hydra is very sensitive to the light, and indeed all seem to be instantly aware of a ray of sunlight. Would it not be curious if it was discovered that the rays which affect sightless creatures, like the hydra, are those about which so much investigation is being carried on?

Many more interesting things might be said about hydras, but these must suffice. I will only add that the more I think about them and the more I see of their habits, the more I realize the truth of the words of Charles Bennet, of Geneva, in Switzerland, about them: "We can only judge of things by comparison, and have taken our ideas of animal life from the larger animals; and an animal we can cut and turn inside out, which we can cut again and it still bears itself well, gives one a singular shock. How many facts are ignored which will come one day to derange our ideas of subjects which we think we understand! At present we just know enough to be aware that we should be surprised at nothing."

A plant of a new species—Bauhinia magalandra—is described as growing on the island of Trinidad, the flowers of which depend on bats for their fertilization. The bat visits the flowers for the insects they harbor, and, in trying to reach them, disturbs the stamens and shakes the pollen from them.