Part 18

Circulation of the Blood.--In the vertebrate animals the blood is said to circulate in the body, because it passes through a more or less closed system of tubes in its course around the body. In the fishes the heart is a two-chambered muscular organ, a thin-walled _auricle_, the receiving chamber, leading into a thick-walled muscular _ventricle_ from which the blood is forced out. The blood is pumped from the heart to the gills; there it loses some of its carbon dioxide; it then passes on to other parts of the body, eventually breaking up into very tiny tubes called _capillaries_. From the capillaries the blood returns, in tubes of gradually increasing diameter, toward the heart again. The body cells lie between the smallest branches of the capillaries. Thus they get from the blood food and oxygen and return to the blood the wastes resulting from oxidation within the cell body. During its course some of the blood passes through the kidneys and is there relieved of part of its nitrogenous waste. Circulation of blood in the body of the fish is rather slow. The temperature of the blood being nearly that of the surrounding media in which the fish lives, the animal has incorrectly been given the term "cold-blooded."

Nervous System.--As in all other vertebrate animals, the brain and spinal cord of the fish are partially inclosed in bone. The central nervous system consists of a _brain_, with nerves connecting the organs of sight, taste, smell, and hearing, and such parts of the body as possess the sense of touch; a _spinal cord_; and _spinal nerves_. Nerve cells located near the outside of the body send in messages to the central system, which are there received as sensations. Cells of the central nervous system, in turn, send out messages which result in the movement of muscles.

Skeleton.--In the vertebrates, of which the bony fish is an example, the skeleton is under the skin, and is hence called an _endoskeleton_. It consists of a bony framework, the vertebral column which protects the spinal cord and certain attached bones, the ribs, with other spiny bones to which the unpaired fins are attached. The paired fins are attached to the spinal column by two collections of bones, known respectively as the _pectoral_ and _pelvic girdles_. The bones in the main skeleton serve in the fish for the attachment of powerful muscles, by means of which locomotion is accomplished. In most fishes, the _exoskeleton_, too, is well developed, consisting usually of scales, but sometimes of bony plates.

Food of Fishes.--We have already seen that in a balanced aquarium the balance of food was preserved by the plants, which furnished food for the tiny animals or were eaten by larger ones,--for example, snails or fish. The smaller animals in turn became food of larger ones. The nitrogen balance was maintained through the excretions of the animals and their death and decay.

The marine world is a great balanced aquarium. The upper layer of water is crowded with all kinds of little organisms, both plant and animal. Some of these are microscopic in size; others, as the tiny crustaceans, are visible to the eye. On these little organisms some fish feed entirely, others in part. Such are the menhaden[33] (bony, bunker, mossbunker of our coast), the shad, and others. Other fishes are bottom feeders, as the blackfish and the sea bass, living almost entirely upon mollusks and crustaceans. Still others are hunters, feeding upon smaller species of fish, or even upon their weaker brothers. Such are the bluefish, squeteague or weakfish, and others.

Footnote 33: It has been discovered by Professor Mead of Brown University that the increase in starfish along certain parts of the New England coast was in part due to overfishing of menhaden, which at certain times in the year feed almost entirely on the young starfish.

What is true of salt-water fish is equally true of those inhabiting our fresh-water streams and lakes. It is one of the greatest problems of our Bureau of Fisheries to discover this relation of various fishes to their food supplies so as to aid in the conservation and balance of life in our lakes, rivers, and seas.

Migration of Fishes.--Some fishes change their habitat at different times during the year, moving in vast schools northward in summer and southward in the winter. In a general way such migrations follow the coast lines. Examples of such migratory fish are the cod, 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 Egg-laying Habits of the Bony Fishes.--The eggs of most bony fishes are laid in great numbers, varying from a few thousand in the trout to many hundreds of thousands in the shad and several millions in the cod. The time of egg-laying is usually spring or early summer. At the time of spawning the male usually deposits milt, consisting of millions of sperm cells, in the water just over the eggs, thus accomplishing fertilization. Some fishes, as sticklebacks, sunfish, toadfish, etc., make nests, but usually the eggs are left to develop by themselves, sometimes attached to some submerged object, but more frequently free in the water. In some eggs a tiny oil drop buoys up the egg to the surface, where the heat of the sun aids development. They are exposed to many dangers, and both eggs and developing fish are eaten, not only by birds, fish of other species, and other water inhabitants, but also by their own relatives, and even parents. Consequently a very small percentage of eggs ever produce mature fish.

The Relation of the Spawning Habits to Economic Importance of Fish.--The spawning habits of fish are of great importance to us because of the economic value of fish to mankind, not only directly as a food, but indirectly as food for other animals in turn valuable to man. Many of our most desirable food fishes, notably the salmon, shad, sturgeon, and smelt, pass up rivers from the ocean to deposit their eggs, swimming against strong currents much of the way, some species leaping rapids and falls, in order to deposit their eggs in localities where the conditions of water and food are suitable, and the water shallow enough to allow the sun's rays to warm it sufficiently to cause the eggs to develop. The Chinook salmon of the Pacific coast, the salmon used in the Western canning industry, travels over a thousand miles up the Columbia and other rivers, where it spawns. The salmon begin to pass up the rivers in early spring, and reach the spawning beds, shallow deposits of gravel in cool mountain streams, before late summer. Here the fish, both males and females, remain until the temperature of the water falls to about 54 deg. Fahrenheit. The eggs and milt are then deposited, and the old fish die, leaving the eggs to be hatched out later by the heat of the sun's rays.

Need of Conservation.--The instinct of this and other species of fish to go into shallow rivers to deposit their eggs has been made use of by man. At the time of the spawning migration the salmon are taken in vast numbers, for the salmon fisheries net over $16,000,000 annually.

But the need for conservation of this important national asset is great. The shad have within recent time abandoned their breeding places in the Connecticut River, and the salmon have been exterminated along our eastern coast within the past few decades. It is only a matter of a few years when the Western salmon will be extinct if fishing is continued at the present rate. More fish must be allowed to reach their breeding places. To do this a closed season on the rivers of two or three days out of each seven while the shad or the salmon run would do much good.

The sturgeon, the eggs of which are used in the manufacture of the delicacy known as _caviar_, is an example of a fish that is almost extinct in this part of the world. Other food fish taken at the breeding season are also in danger.

Artificial Propagation of Fishes.--Fortunately, the government through the Bureau of Fisheries, and various states by wise protective laws and by artificial propagation of fishes, are beginning to turn the tide. Certain days of the week the salmon are allowed to pass up the Columbia unmolested. Closed breeding seasons protect our trout, bass, and other game fish, also the catching of fish under a certain size is prohibited.

Many fish hatcheries, both government and state, are engaged in artificially fertilizing millions of fish eggs of various species and protecting the young fry until they are of such size that they can take care of themselves, when they are placed in ponds or streams. This artificial fertilization is usually accomplished by first squeezing out the ripe eggs from a female into a pan of water; in a similar manner the milt or sperm cells are obtained, and poured over the eggs. The eggs are thus fertilized. They are then placed in receptacles supplied with running water and left to develop under favorable conditions. Shortly after the egg has segmented (divided into many cells) the embryo may be seen developing on one side of the egg. The rest of the egg is made up of food or yolk, and when the baby fish hatches it has for some time the yolk attached to its ventral surface. Eventually the food is absorbed into the body of the fish. The development of the fish is direct, the young fish becoming an adult without any great change in form. The young fry are kept under ideal conditions until later, when they are shipped, sometimes thousands of miles, to their new homes.

NOTE TO TEACHER.--It is suggested that in the spring term the frog be studied, but if animal biology be taken up during the fall term the fish only might be used.

THE FROG

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 surroundings, their color appears to give almost perfect protection. In some species of frogs the color of the skin changes with the surroundings of the frog, another means of protection.

Adaptations for life in the water are numerous. The ovoid body, the head merging into the trunk, the slimy covering (for the frog is provided, 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, or third eyelid, called the _nictitating membrane_, 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. The long flexible tongue, which is fastened at the front, is used to catch insects. 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 well-developed 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, 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.

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_. The latter 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.

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 contains 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 some blood containing much and some containing 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.

Change of Form in Development of the Frog.--Not all vertebrates develop directly into an adult. The frog, for example, changes its form completely before it becomes an adult. This change in form is known as a _metamorphosis_. Let us examine the development of the common leopard frog.

The eggs of this 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 deposited at a 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 upper side of the egg is dark, the light-colored side being weighted down with a supply of yolk (food). The fertilized egg soon segments (divides into many cells), and in a few days, if the weather is warm, these eggs have each 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 water weed by means of a pair of suckerlike projections; later a mouth is formed, and the tadpole begins to feed upon algae or other tiny water plants. At this time, about two weeks after the eggs were laid, gills are present on the outside of the body. Soon after, the external gills are replaced 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 first, although for a time locomotion is performed by means of the tail. In the leopard frog the change from the egg to adult is completed in one summer. In late July or early August, the tadpole begins to eat less, the tail becomes smaller (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.

Shortly after the legs appear, the gills begin to be absorbed, and lungs take their place. At this time the young animal may be seen coming to the surface of the water for air. Changes in the diet of the animal also occur; the long, coiled intestine is transformed into a much shorter one. The animal, now insectivorous in its diet, becomes provided with tiny teeth and a mobile tongue, instead of keeping the horny jaws used in scraping off algae. After the tail has been completely absorbed and the legs have become full grown, there is no further structural change, and the metamorphosis is complete.

Development of Birds.--The white of the hen's egg is put on during the passage of the real egg (which is in the yolk or yellow portion) to the outside of the body. Before the egg is laid a shell is secreted over its surface. If the fertilized egg of a hen be broken and carefully examined, on the surface of the yolk will be found a little circular disk. This is the beginning of the growth of an _embryo_ chick. If a series of eggs taken from an incubator at periods of twenty-four hours or less apart were examined, this spot would be found at first to increase in size; later the little embryo would be found lying on the surface. Still later small blood vessels could be made out reaching into the yolk for food, the tiny heart beating as early as the second day of incubation. After about three weeks of incubation the little chick hatches; that is, breaks the shell, and emerges in almost the same form as the adult.

Development of a Mammal.--In mammals after fertilization the egg undergoes development within the body of the mother. Instead of blood vessels connecting the embryo with the yolk as in the chick, here the blood vessels are attached to an absorbing organ, known as the _placenta_. This structure sends branch-like processes into the wall of the _uterus_ (the organ which holds the embryo) and absorbs nourishment and oxygen by osmosis from the blood of the mother. After a length of time which varies in different species of mammals (from about three weeks in a guinea pig to twenty-two months in an elephant), the young animal is expelled by muscular contraction of the uterus, or is born. The young, usually, are born in a helpless condition, then nourished by milk furnished by the mother until they are able to take other food. Thus we see as we go higher in the scale of life fewer eggs formed, but those few eggs are more carefully protected and cared for by the parents. The chances of their growth into adults are much greater than in the cases when many eggs are produced.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American Book Company. Bigelow, _Introduction to Biology_. The Macmillan Company. Cornell _Nature Study Leaflets_. Bulletins XVI, XVII. Davison, _Practical Zoology_, pages 185-199. American Book Company. Hodge, _Nature Study and Life_, Chaps. XVI, XVII. Ginn and Company. Sharpe, _Laboratory Manual_, pp. 195, 204-209. American Book Company.

ADVANCED

Dickerson, _The Frog Book_. Doubleday, Page and Company. Holmes, _The Biology of the Frog_. The Macmillan Company. Jordan, _Fishes_. Henry Holt and Company. Morgan, _The Development of the Frog's Egg_. The Macmillan Company. Needham, _General Biology_. Comstock Publishing Company.

XVII. HEREDITY, VARIATION, PLANT AND ANIMAL BREEDING

_Problems.--To determine what makes the offspring of animals or plants tend to be like their parents._ _To determine what makes the offspring of animals and plants differ from their parents._ _To learn about some methods of plant and animal breeding._ _(a) By selection._ _(b) By hybridizing._ _(c) By other methods._ _To learn about some methods of improving the human race._ _(a) By eugenics._ _(b) By euthenics._

SUGGESTIONS FOR LABORATORY WORK