Draw the medusa twice natural size, showing all the above parts neatly labeled.

Development. The egg of the medusa after fertilization undergoes a number of changes. First the egg splits in two, then four, eight, and ultimately a mass of cells. This process is known as segmentation. These cells form a hollow ball of cells and swim through the water by means of cilia. Ultimately this little animal settles down on one end and becomes fixed to a rock, seaweed, or pile. The free end becomes indented in the same manner as a hollow rubber ball may be pushed in on one side. This indented side becomes a mouth, tentacles develop around the orifice, and we have an animal that looks very much like the hydra. This animal, now known as a hydroid polyp, buds rapidly and soon forms a colony of little polyps, each of which is connected with its neighbor by a hollow food tube. The hydroid polyp differs from its fresh-water cousin, the hydra, by

usually possessing a tough covering which is not alive.

(Material put

Hydroid Colony (Pennaria).' up in formol may be handed out to the class in small vials). In the portion of the colony you have, where are the polyps located? Examine a single polyp and make out all you can regarding (a) its general form, (b) the position of tentacles, (c) the position of the mouth. The buds which form the free-swimming medusæ are frequently found budding out of the wall of the polyp. Can you describe them?

Alternation of Generations in Cœlenterates. -The lives of a hydroid and a medusa are seen thus to be intimately connected with each other. A hydroid colony produces new polyps by budding. This we know is an asexual method of reproduction. There come from this hydroid colony, however, little buds which give rise to medusæ. These

A hydroid colony of six polyps; f, feeding polyp; r, reproductive polyp; m, a medusa; y, young polyp.

medusæ produce eggs and sperms. Their reproduction is sexual, as was the reproduction by means of eggs and sperms from the prothallus of the fern. So we have in animals, as well as in plants, an alternation of generations.

SEA ANEMONE. Those who have visited our New England coast are familiar with another colenterate called the sea anemone. This animal gets its name from the fact that, seen in a little rocky pool along the shore, 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

Sea anemone.

About natural size. The right-hand specimen is expanded. Note the mouth surrounded by the tentacles. The left-hand specimen is contracted. From model at the American Museum of Natural History.

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. When disturbed or irritated, the animal, like the hydra, contracts into a slimy ball.

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 the outer wall toward the middle

of the cavity. Part of the cavity, as in the hydra, is given up to digesting the food. Food, which is often taken alive into the body, is killed by means of stinging cells found in the long threadlike tentacles developed near the base of the cavity."

A branching madreporic coral.

the surface. Do you find little partitions in these holes?

These cuplike depressions were once occupied by the coral animals or polyps, each in its own cup. The mesenteries of the coral polyp are double and hollow on the under surface. The partitions seen in the coral cups lie between these 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. ASEXUAL REPRODUCTION.-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. in a living state, is alive only on the surface, the polyps building outward on the skeleton formed by their predecessors.

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

Coral,

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

1 See Hunter and Valentine, Manual, page 157.

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 socalled 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 low water 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 cocoanut was washed ashore; thus plant life became established in the atoll and a new outpost to support human life was thus established.

CLASSIFICATION OF CELENTERATES

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

CLASS II. Scyphozoa. Examples: large jellyfishes.

CLASS III.

Actinozoa. Mesenteries present in body cavity. Examples: sea anemones and corals.

Agassiz, A First Lesson in Natural History. D. C. Heath and Company.
Dana, Coral and Coral Islands. Dodd, Mead, and Company.

Parker, Elementary Biology. The Macmillan Company.

XVI. THE STARFISH AND ITS ALLIES

Structure of a Starfish. A glance at the body of a starfish shows us that the name is rightly given. The body is called the disk; the five radiating structures the arms or rays. The term echinoderm (meaning spinyskinned) is also an apt one, as an examination of the dried specimen before us will show. The skeleton of

Ventral or under surface of the starfish. The dark circle in the middle is the mouth, from which radiate the five ambulacral grooves, each filled with four rows of tube feet. Photograph half natural size, by Davison.

The five grooves, which lead outward along the rays from the area around the mouth, are known as the ambulacral grooves because they contain the ambulacræ or tube feet. In the dried specimen the tube feet may be found as very small dried projections in the grooves. There are four rows of tube feet in each groove. Estimate the number of tube feet in a single row and thus figure out the number of tube feet in a starfish. Do you believe the number to be exactly the same for every starfish? Give reasons for your answer. Locomotion in the starfish is performed by the movement of hundreds of the little suckerlike feet. The process of movement of a single tube foot is a complicated one. It is performed partly by means of muscles, but chiefly by means of the passage of water through a system of water tubes within the body of the animal.

1 See Hunter and Valentine, Manual, page 147.