In some members of this group the young are born alive. Sharks, rays, and skates are elasmobranchs.

2. Ganoids.

The bodies of these are ganoids protected by a series of platelike scales of considerable strength. These fishes are the only remnant of what once was the most powerful group of animals on the earth, the great armored fishes of the Devonian

4. The Dipnoi, or Lung Fishes. - This is a very small group, in many respects more like amphibians than fishes, the swim bladder being used as a lung. They live in tropical Africa, South America, and Australia, inhabiting the rivers and lakes there. They withstand drying up in the mud during the dry season, lying dormant for long periods of time in a ball of mud and waking to active life again when the mud coat is removed by immersion in water.

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual. American Book Company.

Davison, Practical Zoology, pages 185-199. American Book Company.

Herrick, Textbook in General Zoology, Chap. XIX. American Book Company. Jordan, Kellogg, and Heath, Animal Studies, XIV. D. Appleton and Company.

ADVANCED

Jordan and Evermann, American Food and Game Fishes. Doubleday, Page, and Company.

Jordan, Fishes. Henry Holt and Company.

Kingsley, Textbook of Vertebrate Zoology. Henry Holt and Company.

Riverside Natural History. Houghton, Mifflin, and Company.

Amphibia. The Frog

Problem XXXVII. Some adaptations in a living frog. (Laboratory Manual, Prob. XXXVII)

Adaptations for Life. The most common frog in the eastern part of the United States is the leopard frog. It is recognized by its greenish brown body with dark spots, each spot being outlined in a lighter colored background. In spite of the apparent lack of harmony with their sur

The leopard frog.

vided, like the fish, with mucus cells in the skin), and the powerful legs with webbed feet, are all evidences of the life which the frog leads.

Locomotion. You will notice that the appendages have the same general position on the body and same number of parts as do your own (upper arm, forearm, and hand; thigh, shank, and foot, the latter much longer relatively than your own). Note that while the hand has four fingers, the foot has five toes, the latter connected by a web. In swimming the frog uses the stroke we all aim to make when we are learning to swim. Most of the energy is liberated from the powerful backward push of the hind legs, which in a resting position are held doubled up close to the body. On land, locomotion may be by hopping or crawling.

Sense Organs. The frog is well provided with sense organs. The eyes are large, globular, and placed at the side of the head. When they are closed, a delicate fold, called the nictitating membrane (or third eyelid), is drawn over each eye. Frogs probably see best moving objects at a few feet from them. Their vision is much keener than that of the fish. The external ear (tympanum) is located just behind the eye on the side of the body. Frogs hear sounds and distinguish various calls of their own kind, as is proved by the fact that frogs recognize the warning notes of their mates when any one is approaching. The inner ear also has to do with balancing the body as it has in fishes and other vertebrates. Taste and smell are probably not strong sensations in a frog or toad. They bite at moving objects of almost any kind when hungry. Experience has taught these animals that moving things, insects, worms, and the like, make good food. These they swallow whole, the tiny teeth being used to hold the food. Touch is a welldeveloped sense. They also respond to changes in temperature under water, remaining there in a dormant state for the winter when the temperature of the air becomes colder than that of the water.

Breathing. The frog breathes by raising and lowering the floor of the mouth, pulling in air through the two nostril holes. Then the little flaps over the holes are closed, and the frog swallows this air, thus forcing it down into the baglike lungs. The skin is provided with many tiny blood vessels, and in winter, while the frogs are dormant at the bottom of the ponds, it serves as the only organ of respiration.

Although we shall take up the study of the internal structure of

the frog more in detail when we discuss the structure and uses of the parts of the body in man, we may now learn something of the position and use of some of the structures found within the body cavity.

The Food Tube and its Glands. The mouth leads like a funnel into a short tube, the gullet. On the lower floor of the mouth can be seen the slitlike glottis leading to the lungs. The gullet widens almost at once into a long stomach, which in turn leads into a much

coiled intestine. This widens abruptly at the lower end to form the large intestine. This in turn leads into the cloaca (Latin, sewer) into which open the kidneys, urinary bladder, and reproductive organs (ovaries or spermaries). Several glands, the function of which is to produce digestive fluids, open into the food tube. These digestive fluids, by means of the ferments or enzymes contained in them, change insoluble food materials into a soluble form. This allows of the absorption of food material through the walls of the food tube into the blood. The glands (having the same names and uses as those in man) are the salivary glands, which pour their juices into the mouth, the gastric glands in the walls of the stomach, and the liver and pancreas, which open into the intestine. (See Digestion, pages 352-365.)

Circulation. -The frog has a well-developed heart, composed of a thick-walled muscular ventricle and two thin-walled auricles. The heart pumps the blood through a system of closed tubes to all parts of the body. Blood enters the right auricle from all parts of the body; it then con

tains considerable carbon dioxide; the blood entering the left auricle comes from the lungs, hence it contains a considerable amount of oxygen. Blood leaves the heart through the ventricle, which thus pumps blood containing much and little oxygen. Before the blood from the tissues and lungs has time to mix, however, it leaves the ventricle and by a delicate adjustment in the vessels leaving the heart most of the blood containing much oxygen is passed to all the various organs of the body, while the blood deficient in oxygen, but containing a large amount of carbon dioxide, is pumped to the lungs, where an exchange of oxygen and carbon dioxide takes place by osmosis.

In the tissues of the body wherever work is done the process of burning or oxidation must take place, for by such means only is the energy necessary to do the work released. Food in the blood is taken to the muscle cells or other cells of the body and there oxidized. The products of the burning carbon dioxide and any other organic wastes given off from the tissues must be eliminated from the body. As we know, the carbon dioxide passes off through the lungs and to some extent through the skin of the frog, while the nitrogenous wastes, poisons which must be taken from the blood, are eliminated from it in the kidneys. Thus wastes are passed off from the body.

Problem XXXVIII. The development of a frog. (Laboratory Manual, Prob. XXXVI)

(a) Conditions favorable.

(b) Metamorphosis.

(c) Development of a toad (optional).

Field and Home Work. During the first warm days in March or April, look for gelatinous masses of frogs' eggs attached to sticks or water weed in shallow ponds. Collect some and try to hatch them out in a shallow dish in the window at home. Make experiments to learn whether temperature affects the development of the egg in any way. Place eggs

in dishes of water in a warm room and in a cold room, also some in the ice box. Make observations for several weeks as to rate of development of each lot of eggs. Also try placing a large number of eggs in one dish, thus cutting down the supply of available oxygen, and in another dish near by, under the same conditions of light and heat, place a few eggs. Do both batches of eggs develop with the same rapidity? In all these experiments be sure to use eggs from the same egg mass, so as to make sure that all are of the same age.

Development.-The eggs of the leopard frog are laid in shallow water in the early spring. Masses of several hundred, which may be found attached to twigs or other supports under water, are de