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bristles called dsete may be found. Determine if these rows are single or double. How many setæ to a segment?
Every segment except the first three and the last is 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 your 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. Make a drawing of several segments to show the arrangement of setæ.
Notice that 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.
Diagram to show how movement of a seta is accomplished; M, muscles; S, seta; W, body wall. (After Sedgwick and Wilson.)
Test a worm by placing half in and half out of a darkened box. does it seem to prefer, light or darkness? There are no eyes visible. careful study of the worm with the microscope, however, has revealed the fact that scattered through the skin of the anterior segments are many little structures which not only 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 but slightly to sound, the sense of touch is well developed in all parts of the body. Notice especially how the worm uses the whole anterior end to feel with. Jar the dish in which the worm rests lightly, and note the reaction that takes place.
Feeding Habits.- Worms may be kept in the laboratory for some time in a glass dish or box filled with soil. They feed on pieces of lettuce or cabbage leaf. A feeding worm will show the
Forepart of an earthworm with the left body wall removed; a, dorsal blood vessel; b, brain; c, crop; g, gizzard; i, intestine; k, nephridia; m, mouth; n, one of the ganglia of the nerve cord; oe, esophagus; p, pharynx; v, ventral blood vessel. Davison, Zoology.
proboscis, an extension of the upper lip which is used to push food into the mouth. The earthworm is not provided with hard jaws or teeth. Yet it literally eats its way through the hardest HUNTER'S BIOL.-14
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 any small pebble or leaf, 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
Diagrammatic cross section of the body of a cœlenterate A, and that of a worm B.
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, and that ten tons of soil might pass through their bodies in a single year to be brought to the surface. Earthworms, in spite of their fondness for some garden vegetables and young roots, do an immense amount of good by breaking up the soil, thus allowing water and oxygen to penetrate to the roots of plants.
Comparison between Hydra 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. The digested food material passes by osmosis into the body cavity. Some food reaches the blood vessels and is pumped over the body, most of it is used by the organs which it bathes. Nitrogenous waste is excreted from each segment through a pair of coiled tubules called nephridia.
Course of Blood. - In a large earthworm the course of the blood is easy to follow. Notice the position of the dorsal blood vessel. Watch it at one point. The blood vessel expands as the
blood passes slowly forward. The whole dorsal blood vessel, and especially several tubes which connect it with a ventral vessel on the opposite side, are contractile. These connecting tubes are known as hearts, because they serve to pump the blood in a definite direction. Compare the rate of pulsation with your own pulse.
Respiration. 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. Respiration in the earthworm is simply the exchange or osmosis of gases through the skin, the oxygen passing into the blood, the carbon dioxide formed from the oxidation taking place within
the body of the worm passing out.
DEVELOPMENT. Notice in some worms the swollen area (about one third the distance from the anterior end) called the girdle or clitellum. This area 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 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. If possible, procure some young worms and compare them with older ones.
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 clitellum 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 A marine worm (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 most of the time in tubes in
the sand; these tubes are constructed of slime excreted from the body of the worm.1
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.
Unsegmented Worms. Some worms are unsegmented; 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. Of much interest to us is the life history of the flatworm infesting the liver of sheep, causing the disease called liver rot,
A flatworm (Yungia Aurantiaca), much magnified. From model in the American Museum of Natural History.
which causes annually a loss of several millions of dollars' worth of sheep. This worm is called the liver fluke because of its abode in the liver of the sheep. The developing eggs pass out from the liver into the intestine and thence outside of the body. If the egg happens to be deposited in water, it develops, otherwise it dies. The embryo is a little oval, ciliated creature, microscopic in size. This embryo swims about until it reaches a water snail. Here it lives as a parasite, loses its cilia, becomes larger, and gives rise to a number of little larvæ called rediæ. The rediæ give rise to more larvæ, some like themselves and others tadpole-shaped. The latter larvæ leave the snail, swim
1 If the living sandworm is obtainable, a laboratory period may be devoted to its activities. See Hunter and Valentine, Manual, page 133.
to the shore of the pond, and encyst themselves in the grass near its border. If this grass is eaten by sheep, the encysted larvæ (called cercaria) are taken
into the digestive tract and then develop into adult flukes.
Cestodes or Tapeworms.These parasites infest man and many other vertebrate animals. The tapeworm (Tania solium) passes through two stages in its life history, the first within a pig, the second within the intestine of man. The eggs of the worm are taken in with the pig's food. The worm develops within the
intestine of the pig, but Development of the liver fluke; A, ciliated larva;
soon makes its way into the muscles. If man eats pork containing these worms, he
B, sporocyst, containing new sporocyst (r), and redia (m.); C, redia, containing daughter redia and tadpolelike cercaria; D, fully developed cercaria.
may become a host for the tapeworm. The animal, which at this stage consists of a round headlike part provided with hooks, fastens itself to the wall of the intestine. This head now buds off a series of segmentlike structures, which are practically bags full of eggs. These structures, called proglottids, break off from time to time, thus allowing the eggs to escape. The proglottids have no separate digestive systems, but the whole body surface, bathed in digested food, absorbs it and is thus enabled to grow rapidly.
Roundworms. Still other wormlike creatures called roundworms are of importance to man. Some, as the vinegar eel found in vinegar, or the pinworms parasitic in the lower intestine of man, do little or no harm. The pork worm or Trichina, however, is a parasite which may cause serious injury. It passes through the first part of its existence as a parasite in a pig or other vertebrate (dog, cat, ox, or horse), where it encysts itself in the muscles