shortening one portion of the body after another. On the ventral region of the body are rows of bristles which aid in locomotion. The bristles project backward when the worm is moving forward, and so keep the animal from slipping backward when it lengthens itself. The bristles also serve to hold the animal in its burrow.

Earthworms are of considerable value in the soil. They burrow through the earth by swallowing the dirt which is mixed with vegetable matter; both are then acted upon by digestive juices in the alimentary canal. The refuse of the food, which is not available for use in the body, is ejected from the posterior end of the intestine. The little piles of dirt that are sometimes so common on a lawn are the "castings" of earthworms. It has been found that soil worked over by these animals is in better condition for the growth of plants. Then, too, the deeper soil that has not been used by plants is brought to the surface and mingled with the dirt recently used. Darwin 1 estimated that in England earthworms annually bring to the top of the ground eighteen tons of soil per acre.

135. Suggestions for the study of the earthworm. study.

Laboratory

This study should be made upon living worms. The pupil should first note and describe the general shape and segmentation of the animal, the differences between the anterior and posterior ends, the dorsal and ventral surfaces, and the characteristic appearance of the girdle. An earthworm should be placed on a moist surface such as soil or wet paper, and the locomotion of the animal observed and described. A large specimen should be pulled, anterior end first, between the fingers, and the action of the bristles noted and their situation and appearance studied with the help of a magnifier. Touch the earthworm on various parts of the body, and determine, if possible, which portions are the most sensitive. Look on the dorsal and ventral surfaces for blood vessels, and watch the pulsations of the blood in these vessels; describe the location of these blood vessels and state the direction in which blood flows in each of them.

'Darwin's "Vegetable Mold and Earthworms."

136. Relatives of the earthworm.

- Two forms of animals that

formerly were classed with the earthworm under the head of "worms" are the tapeworm (Fig. 129) and trichina. The tape worm is sometimes present in beef and trichina

137. Fresh water mussel. - The fresh water mussels are mollusks that are sometimes called clams. They are often quite abundant on the bottom of creeks, rivers, ponds, or lakes. Usually they are partly covered with sand or mud, sometimes even more than is shown in Figure 131. It will be

seen at once that the A, head of

mussel is inclosed by a shell. This consists of two parts called valves;

tapeworm FIG. 129.

B. tapeworm, about 15 feet long,
omitted portions being indicated

The tapeworm. (Shipley and
MacBride.)

hence these animals, as well as salt water mussels, clams, and oysters are called bivalves (Latin bis = two + valve). The two valves are held together along one margin by a tough material that serves as a hinge. On each valve near the hinge, a prominence, known as the beak or umbo, may be readily seen. Around

the umbo, in ever widening concentric rings, are the lines of growth of the animal, which indicate younger stages in its development. Let us now pull up a mussel and lay it on a sandy bottom. In a few moments the shell will open somewhat and from one end will project a pinkish body, which may finally extend some distance.

exhalent siphon.

Now if one is patient, and the animal feels at home, it will be possible to see the method of eating and breathing. At the end opposite the foot there may slightly project from the shell a fringed and somewhat tubular-shaped structure. Let us place a little finely powdered carmine in the water above the opening. As the carmine slowly sinks and comes opposite the tube, the particles will suddenly be drawn into the tube. This shows that water is being sucked into the tube, and it brings with it oxygen and any food that may be near, such as microscopic plants and animals.

umbo

foot

FIG. 131. Mussel burrowing in sand.

To learn any more about the feeding and breathing of the mussel it will be necessary to open the shell. Let us take 'another mollusk and pry open the valves. We shall soon find that this is not easy to do. The reason will be evident after studying Figure 132.

So we pry the

The valves are held together by strong muscles. valves open a little with a heavy knife and then slip another sharp knife in close to the valve, where we meet an obstruction toward one end. When we have cut this, the valve opens at that

end. After cutting the muscle at the other end, we can readily separate the valves. All over the surface of the animal, except where the two muscles were attached to the shell, is a thin covering called the mantle. By raising the body of the mussel from the valve it will be evident that there is a similar structure on the other side.

Now, if we fold back the mantle, it will be possible to follow the course of the food and water. The first thing that strikes our at

tention is the contracted foot, and above this is a soft mass called the abdomen. In the abdomen are found the digestive organs. On each side of the abdomen are two broad, thin flaps, the gills, by which the animal breathes. Between the foot and the end that was buried in the sand are found, on either side of the body, two small flaps or palps, and between them lies the mouth opening. To this mouth the food that has been swept into the tube is brought by the waving of thousands of cilia that are found on the surface cells of the gills and palps.

Let us now return to the study of the mussel partly covered by the sand. The hinge is on the dorsal region of the body, the free edges of the valves on the ventral, while the mouth and foot are at

the anterior end. Hence, the animal in its natural position "stands on its head," or at least where its head ought to be. From the posterior end projects the tubular structure to which reference has been made.

Let us again drop some powdered carmine closer to the animal, and watch the particles when they reach a point just above the tube where we saw the particles enter. We shall now see the carmine carried away from the animal instead of into it. A closer examination reveals the fact that the tubular structure has a second opening above the first. Both of these tubes are called siphons, the lower being the incurrent siphon, and the upper the excurrent siphon. The stream of water forced out of the excurrent siphon carries with it the carbon dioxid and other wastes of the body.

138. Suggestions for study of the mussel. - It is desirable to have students see the mussel in its natural home. They should tell where they found the animals and the positions in which they were seen. It would then be well for the pupil to study in the laboratory the shell, making out the points of structure described above. A drawing of a side view of the mussel should be made and labeled as follows: valve, umbo, hinge, lines of growth, anterior region, posterior region, dorsal edge, ventral edge. It is also desirable that a drawing of the animal in the sand or mud be made and the incurrent and excurrent siphon openings be labeled.

The pupil might well follow the account as given above, verifying the statements and experiments, and making drawings of the mussel with the shell open and all the animal lying in one valve. Label: mantle, muscles that close shell, incurrent siphon, excurrent siphon. Also a drawing should be made of the mussel with the mantle removed. Label: foot, abdomen, palps, mouth, gills. Write an account of how the mussel moves or burrows, how it feeds and breathes.

- Some of the relatives of the

139. Relatives of the mussel. mussel are the clams, oysters, salt water mussels, snails (Fig. 133), and slugs. While the fresh water mussels are not much used for