because, when expanded, it looks like a beautiful flower of a golden yellow or red color. The body of the sea anemone is like the hydra, a column attached at one end. The free end is provided with a mouth surrounded with a great number of tentacles. These, when expanded, look like the petals of a flower. The sea anemone is a very voracious flower, for by means of the batteries of stinging cells in its tentacles it is able to catch and devour fishes and other animals almost as large as itself. disturbed or irritated, the animal contracts into a slimy ball, making it difficult to dislodge from its attachment.

When

Although the sea anemone is like a large hydra in appearance, its interior is different. The hollow digestive cavity contains a number of partitions more or less complete, which run from

A branching madreporic coral.

the outer wall toward the middle of the cavity. These partitions, known as mesenteries, are found in pairs. Part of the cavity, as in the hydra, is given up to digesting the food. Food is killed by means of stinging cells found in the long threadlike tentacles.

Coral. — If a piece of madreporic coral is examined with a hand lens, a

A single coral cup, showing the walls of lime built by the mesenteries. From a photograph loaned by the American Museum of Natural History.

Asexual Reproduction.

number of little depressions will be seen in the limy surface, each of which has tiny partitions within it.

These cuplike depressions were once occupied by the coral animals or polyps, each in its own cup. The mesenteries of the coral polyp are paired and hollow on the under surface. The partitions seen in the coral cups lie between the pairs of mesenteries, and are formed by them when the animal is alive. Sea water has a considerable amount of lime in its composition. This lime (calcium carbonate) is taken from the water by certain of the cells of the coral polyp and deposited around the base of the animal and between the mesenteries, thus giving the appearance just seen in the cups of the coral branch.

These polyps reproduce by budding, and when alive cover the whole coral branch with a continuous living mass of

polyps, each connected with its neighbor. In this way great masses of coral are formed. Coral, in a living state, is alive only on the surface, the polyps building outward on the skeleton formed by their predecessors. Economic Importance of Corals. - Only one (astrangia) of a great many different species of coral lives as far north as New York. In tropical waters they are very abundant. Coral building has had and still has an immense influence on the formation of islands, and even parts of continents in tropical seas. Not only are many of the West Indian islands composed largely of coral, but also Florida, Australia, and the islands of the southern Pacific are almost entirely of coral formation.

Coral Reefs. The coral polyp can live only in clear sea water of moderate depth. Fresh water, bearing mud or other impurities, kills them immediately. Hence coral reefs are never found near the mouths of large fresh-water rivers. They are frequently found building reefs close to the shore. In such cases these reefs are called fringing reefs. The so-called barrier reefs are found at greater distance (sometimes forty to fifty miles) from the shore. An example is the Great Barrier Reef of Australia. The typical coral island is called an atoll. It has a circular form inclosing a part of the sea which may or may not be in communication with the ocean outside the atoll. The atoll was perhaps at one time a reef outside a small island. This island disappeared, probably by the sinking of the land. The polyps, which could live in water up to about one hundred and fifty feet, continued to build the reef until it arose to the surface of the ocean. As the polyps could not exist for long above lowwater line, the animals died and their skeletons became disintegrated by the action of waves and air. Later birds brought a few seeds there, perhaps a coconut was washed ashore; thus plant life became established in the atoll, and a new outpost to support human life was established.

CLASSIFICATION OF CELENTERATES

CLASS I. Hydrozoa. Body cavity containing no mesenteries, usually alternation of generation. Examples: Hydra, hydroids.

CLASS II. Scyphozoa.

CLASS III. Actinozoa.

ones and corals. CLASS IV. Ctenophora.

Examples: large jellyfishes.

Mesenteries present in body cavity. Examples: sea anem

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American
Book Company.

Agassiz, A First Lesson in Natural History. D. C. Heath and Company.
Holder, Half Hours with the Lower Animals. American Book Company.
Jordan, Kellogg, and Heath, Animal Studies. D. Appleton and Company.

ADVANCED

Hertwig, R., General Principles of Zoology. Henry Holt and Company.
Miner, A Guide to the Sponge Alcove. Guide Leaflet, No. 23, American Museum

of Natural History, New York.

Parker, Elementary Biology. The Macmillan Company.

Parker and Haswell, Textbook of Zoology. The Macmillan Company.
Sedgwick and Wilson, General Biology. Henry Holt and Company.

Verworn, General Physiology. The Macmillan Company.

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