XVII. THE WORMS, A STUDY OF RELATIONS TO ENVIRONMENT Problem XXVII. The relation of the earthworm to its surroundings (optional). (Laboratory Manual, Prob. XXVII.) Effect of Surroundings on Plants. - Animals as well as plants are influenced very greatly by their surroundings or environment. We have seen how green plants behave toward the various factors of their environment; how heat and moisture start germination in a seed; how the roots grow toward water; how gravity influences the root and the stem, pulling the root downward and stimulating the stem to grow upward; how the stem grows toward the source of light; and how the leaves put their flat surfaces so as to get as much light as possible; and how oxygen is necessary for life to go on. It is quite possible to show that the factors of environment act upon animals as well as plants, although it is much harder to explain why an animal does a certain thing at a certain time. How One-celled Animals respond to Stimuli. We have seen that the single-celled animals respond to certain stimuli in their surroundings. The presence of food attracts them; when they run into an object, they respond immediately by backing away, thus showing that they have a sense of touch. If part of a glass slide containing paramœcia is heated slightly, the animals will respond to the increase in heat by moving toward the cooler end. Many other experiments might be quoted to show that the living matter of a simple animal is sensitive to its surroundings. The Earthworm in its Relation to its Surroundings. The earthworm, familiar to most boys as bait, shows us in many ways how a many-celled animal responds to stimuli. Careful observation. of the body of a living earthworm shows us that its long tapering body is made up of a large number of rings or segments. The number of these segments will be found to vary in worms of different size, the larger worms having more segments. If the two ends of the worm be touched lightly with a small stick or straw, one end will be found to respond much more readily to touch than the other end. The more sensitive end is the front or anterior end, the other end being the posterior end. Jar the dish in which the worm is crawling; it will immediately respond by contracting its body. Living earthworms tend to collect along the sides of a dish or in the corners. This seems to be due to an instinct which leads them to inhabit holes in the ground. An earthworm placed half in and half out of a darkened box soon responds by crawling into the darkened part and remaining there. There are no eyes visible. A careful study of the worm with the microscope, however, has revealed the fact that scattered through the skin, particularly of the anterior segments, are many little structures which not only enable the animal to distinguish between. light and darkness, but also light of low and high intensity, as well as the direction from which it comes. A worm has no ears or special organs of feeling. We know, however, that although a worm responds to vibrations of low intensity, the sense of touch is well developed in all parts of the body. It also responds to the presence of food, as can be proved if bits of lettuce or cabbage leaf are left overnight in a dish of earth where worms are kept. Locomotion of an Earthworm. If we measure an earthworm when it is extended and compare with the same worm contracted, we note a difference in length. This is accounted for when we understand the method of locomotion. Under the skin are two sets of muscles, an outer set which passes in a circular direction around the body, and an inner set which runs the length of the body. The body is lengthened by the contraction of the circular muscles. How might the body be shortened? Diagram to show how movement of a seta is accomplished; The under surface of the worm is provided with four double rows of tiny bristles called w setæ, every segment except the first three and the last being provided with setæ. Each seta has attached to it small muscles, which turn the seta so it may point in the opposite direction from which the worm is moving. If you watch a specimen carefully, you will see that locomotion is accomplished by the thrusting forward of the anterior end; then a wave of muscular contraction passes down the body, thus shortening the body by drawing up the posterior end. The setæ at the anterior end serve as anchors which prevent the body from slipping backward as the posterior end is drawn up. M, muscles; S, seta; W, body wall. (After Sedgwick and Wilson.) How the Worm digs Holes. A feeding worm will show the proboscis, an extension of the upper lip which is used to push food into the mouth. Forepart of an earthworm with the left body wall removed to show the body cavity and food tube within it: m, mouth; p, pharynx, c, g, i, food tube. The earthworm is not provided with hard jaws or teeth. Yet it literally eats its way through the hardest soil. Inside the mouth opening is a part of the food tube called the pharynx. This is very muscular so that it can be extended and withdrawn by the worm. When applied to the surface of the soil, which is first moistened by the worm, it acts as a suction pump and draws it into the food tube. As the worms take organic matter out of the ground as food, they pass the earth through the body in order to get this food. The earth is mixed with fluids poured out from glands in the food tube, and is passed out of the body and deposited on the surface of the ground, in the form of little piles of moist earth. These are familiar sights on all lawns; they are called worm casts. Charles Darwin calculated that fifty-three thousand worms may be found in an acre of ground, that ten tons of soil might pass through their bodies in a single year and thus be brought to the surface, and that they plow more soil than all the farmers put together. Earthworms, in spite of their fondness for some garden vegetables and young roots, do much good by breaking up the soil, thus allowing water and oxygen to penetrate to the roots of plants. Comparison between Hydra Body cavity and Worm. -The digestive tract of the worm is an almost straight tube inside of another tube. The latter is divided by partitions which mark the boundary of each segment. The outer cavity is known as the body cavity. In the hydra no distinction existed between the body cavity and digestive tract. In the animals higher than the cœlenterates the digestive tract and body cavity are distinct. Food is digested within the food tube, is passed through the walls of this tube into the body cavity, and is in part carried by the blood to various parts of the body. No gills or lungs are present, the thin skin acting as an organ of respiration. But the worm is unable to take in oxygen unless the membranelike skin is kept moist. Development. Diagrammatic cross section of the body of a cœlenterate, and that of a worm. Notice in some worms the swollen area called the girdle (about one third the distance from the anterior end). This area periodically forms a little sac in which the eggs of the worm are laid. As it passes toward the anterior end of the worm, it receives from the body openings the sperms and a nutritive fluid in which the eggs live. The fertilized eggs are then left to hatch. The capsules may be found in manure heaps, or under stones, in May or June; they are small yellowish or brown bags about the diameter of a worm. Regeneration. If a one-celled animal be cut into two pieces, each piece, if it contains part of the nucleus, will grow into a whole cell. The hydra, some hydroids, jellyfish, and flatworms, if injured, may grow again parts that are lost. This power is known as regeneration. Earthworms possess to a large degree the power of replacing parts lost through accident or other means. The anterior end may form a new posterior end, while the posterior end must be cut anterior to the girdle to form a new anterior end. This seems to be in part due to the greater complexity of the organs in the anterior end. The Sandworm. Other segmented worms are familiar to some of us. The sandworm, used for bait along our eastern coast, is a segmented worm which lives between tide marks in sandy localities. It is plainly segmented, each segment bearing a pair of locomotor organs called parapodia (meaning side feet). A part of each parapodium is prolonged into a triangular gill. The animal has a distinct head, which is provided with tentacles, palps, and eye spots. The mouth has a pair of hard jaws which may be protruded. In this way the animal seizes and draws prey into its mouth. The sandworm swims near the surface of the water, the body bending in graceful undulations as the parapodia, like little oars, force the worm forward. They spend much of the time in tubes in the sand, which are constructed in part of slime excreted from the body of the worm. The sandworm (nereis). The Leech. The common leech or bloodsucker is a flattened segmented worm, inhabiting fresh-water ponds and rivers. The adult is provided with two sucking disks, by means of which it fastens itself to objects. The mouth is on the lower surface close to the anterior disk. Locomotion is accomplished by swimming or by means of the suckers, somewhat after the manner of a measuring worm. They feed greedily and are often found gorged with blood, which they suck from the body of the victim. Discomfort, but no danger, attends the bite of the bloodsucker, so dreaded by the small boy. Problem XXVIII. A study of some animal associations. (Laboratory Manual, Prob. XXVIII.) Some worms are unseg Some Worms which harm Man. mented; such are the flatworms and roundworms. A common leaflike form of flatworm may be found clinging to stones in our fresh-water ponds or brooks. Most flatworms are, however, parasites on other animals; that is, they obtain food and shelter from some other living creature, but give them no benefits in return. Parasitism is one-sided, the host giving everything, the parasite receiving everything. Consequently, the parasite frequently becomes fastened to its host during adult life A flatworm (Yungia Aurantiaca), much magnified. From model in the American Museum of Natural History. |