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menhaden, herring, and bluefish. The migrations are due to temperature changes, to the seeking after food, and to the spawning instinct. Some fish migrate to shallower water in the summer and to deeper water in the winter; here the reason for the migration is doubtless the change in temperature.
The herring fisheries have always been a source of wealth to the inhabitants of northern Europe. The banks and shallows of the coast of Newfoundland were undoubtedly known to the Norsemen long before the discovery of this country by Columbus. Classification of Fishes. The animals we recognize as fishes are
grouped by naturalists into four groups:
1. THE ELASMOBRANCHS.-These fishes have a skeleton formed of cartilage which has not become hardened with lime. The gills communicate with the surface of the body by separate openings instead of having an operculum. The skin is rough and the eggs few in number. In some members of this group the young are born alive. Sharks, rays, and skates are Elasmobranchs.
2. GANOIDS. - Fish in which the body is 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 age.
A bony fish.
The gar pike is an example.
3. THE TELEOSTS OR BONY FISHES. -They compose ninety-five per cent of all living fishes. In this group the skeleton is bony, the gills are protected by an operculum, and the eggs are numerous. Most of our common food fishes belong to this class.
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.
FOR THE PUPIL
Davison, Practical Zoology, pages 185-199. American Book Company.
Hunter and Valentine, Laboratory Manual of Zoology, page 167. Henry Holt and
Jordan, Kellogg, and Heath, Animal Studies, XIV. D. Appleton and Company.
FOR THE TEACHER
Jordan and Evermann, American Food and Game Fishes. Doubleday, Page, and
Kingsley, Text-book of Vertebrate Zoology. Henry Holt and Company.
The Frog; Body. - In the body of man we find two distinct regions, the head and trunk. Are such regions to be found in the frog? shape of the body fitted for life in the water?
How is the Habitat. There is considerable difference in the habitat of the green frog and the leopard frog. The former may usually be found in ponds or brooks in which considerable vegetation is found. The latter live in pools or woody swamps in which the bottoms are dark from a background of dead leaves or mud. Remembering this, how might the color of the frog harmonize with its surroundings? Is this of advantage to the frog? In what respects?
Protective Resemblance. - Notice the position of the frog at rest in the water. In its natural habitat a frog in this position would scarcely be noticed, so perfect is the resemblance to the surroundings. Notice that
the only parts of the frog that show above the surface of the water are the eyes and that part of the head bearing the nostril holes. Appendages. Compare the anterior limb with your own arm. Identify in each, upper arm, forearm, and hand. Note the number of fingers. In the same manner find the thigh, shank, and foot in the posterior limb. In what respects do ankle and foot differ in the frog and in man? What adaptations for locomotion (swimming) do you find in the frog?
Skin. Notice the slimy skin of the frog. This is due to the presence in the skin of cells which secrete and pour out mucus. Might this slime be of any use to the animal? The skin of the frog is supplied with numerous tiny blood vessels. The blood in these thin-walled tubes gains oxygen from the surrounding atmosphere and from the water, while carbon dioxide is given off. Thus the skin is used in the process of respiration.
The Eye.-The eye of the frog differs somewhat in shape from our own. Note the positions on the side of the head. Touch the eye. How is it protected? Look for a delicate fold, the nictitating membrane (sometimes called the third eyelid), which may be drawn over the eye.
Have you any experience as to the keenness of vision in the frog? Do they jump into the water because they see or hear you? Any experiment which will throw light on this point will make an interesting piece of original work for extra credit. Can you perform any experiment which will show whether the frog prefers light to darkness?
Ear.-The tympanic membrane or eardrum of the frog may be found on the side of the head. It is a circular area of tightly stretched skin. In man the ear drum is beneath the surface of the body near the inner end of the canal or tube which we see in the external ear. In the frog and in man a connection exists between the mouth and the inner surface of the ear drum. This tube is known as the Eustachian tube.
Home Experiment. Give an account of any experiment that you may perform that will prove that frogs can hear, or the nature of sounds that attract their attention.
1 For laboratory exercise see Hunter and Valentine, Manual, page 170. (Either the green frog or the leopard frog may be used for the laboratory suggestions given in this book.)
Note the shape, position, and relatively large size of the mouth in the frog as compared with man. Make a drawing of the frog in a natural position, having all parts visible in the drawing carefully labeled.
Characteristics of Amphibia.-The frog belongs to the class of vertebrates known as the Amphibia. As the name indicates, members of this group pass more or less of their life in the water, although in the adult state they are provided with lungs. In the earlier stages
Full-grown green frog, about half natural size.
of their development they take oxygen into the blood by means of gills. At all times, but especially during the winter, the skin serves as a breathing organ. The skin is soft and unprotected by bony plates or scales. The heart has three chambers, namely, two auricles and one ventricle. Most amphibians undergo a complete metamorphosis.
Life Habits of Green Frog. The green frog inhabits shallow fresh-water ponds, streams, and marshes. Much of the daytime they may be seen sunning themselves. They live to a large extent upon insects, which they catch by protruding their long bi-cleft tongue. They also eat small algae and aquatic animals, and are in fact omnivorous, even eating their own young.
Life History. During the first warm days in March or April, look for gelatinous masses of frog's eggs attached to sticks or waterweed 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 insure all being of the same age.
The eggs of the frog are laid in shallow water in
single laying. Immediately before leaving the body of the female they receive a coating of jellylike material, which swells up after the eggs are laid. Thus they are protected from the attack of fish or other animals which might use them as food. The fertilized egg soon segments (divides into many cells), and in a few days, if the weather is warm, these cells have grown into an oblong body which shows the form of a tadpole. Shortly after the tadpole wriggles out of the jellylike case and begins life outside the egg. At first it remains attached to some waterweed by means of a suckerlike projection; later a mouth is formed at this point and the tadpole begins to feed upon algæ or other tiny water plants. At this time gills are present on the outside of the body. Soon after this, the external gills are re
placed by gills which grow out under a fold of the skin which forms an operculum somewhat as in the fish. Water reaches the gills through the mouth and passes out through a hole on the left side of the body. As the tadpole grows larger, legs appear, the hind legs making their appearance first, although for a long time locomotion is performed by means of the tail. In some species of frogs the changes from the egg to adult are completed in one summer.
Frog's eggs from three to ten hours old. All stages from four cells to thirty-two cells may be noted. From photograph, enlarged four times, by Davison.
A month or two after hatching, the tadpole begins to eat less, the tail is used up rather rapidly (being absorbed into other parts of the body), and before long the transformation from the tadpole to the young frog is complete. In the green frog and bullfrog the metamorphosis is not completed until the beginning of the second summer. The large tadpoles of such forms bury themselves in the soft mud of the pond bottom during the winter.