[Illustration: The bream. _A_, dorsal fin; _B_, caudal fin; _C_, anal fin; _D_, pelvic fin; _E_, pectoral fin.]
The Appendages and their Uses.--The appendages of the fish consist of paired and unpaired fins. The paired fins are four in number, and are believed to correspond in position and structure with the paired limbs of a man. Note the illustration above and locate the paired _pectoral_ and _pelvic_ fins. (These are so called because they are attached to the bones forming the pectoral and pelvic girdles. See page 268.) Find, by comparison with the Figure, the _dorsal_, _anal_, and _caudal_ fins. How many unpaired fins are there?
The flattened, muscular body of the fish, tapering toward the caudal fin, is moved from side to side with an undulating motion which results in the forward movement of the fish. This movement is almost identical with that of an oar in sculling a boat. Turning movements are brought about by use of the lateral fins in much the same way as a boat is turned. We notice the dorsal and other single fins are evidently useful in balancing and steering.
The Senses.--The position of the eyes at the side of the head is an evident advantage to the fish. Why? The eye is globular in shape. Such an eye has been found to be very nearsighted. Thus it is unlikely that a fish is able to perceive objects at any great distance from it. The eyes are unprotected by eyelids, but the tough outer covering and their position afford some protection.
Feeding experiments with fishes show that a fish becomes aware of the presence of food by smelling it as well as by seeing it. The nostrils of a fish can be proved to end in little pits, one under each nostril hole. Thus they differ from our own, which are connected with the mouth cavity. In the catfish, for example, the _barbels_, or horns, receive sensations of smell and taste. They do not perceive odors as we do for a fish perceives only substances that are dissolved in the water in which it lives. The senses of taste and touch appear to be less developed than the other senses.
Along each side of most fishes is a line of tiny pits, provided with sense organs and connected with the central nervous system of the fish. This area, called the _lateral line_, is believed to be sensitive to mechanical stimuli of certain sorts. The "ear" of the fish is under the skin and serves partly as a balancing organ.
Food Getting.--A fish must go after its food and seize it, but has no structures for grasping except the teeth. Consequently we find the teeth small, sharp, and numerous, well adapted for holding living prey. The tongue in most fishes is wanting or very slightly developed.
Breathing.--A fish, when swimming quietly or when at rest, seems to be biting when no food is present. A reason for this act is to be seen when we introduce a little finely powdered carmine into the water near the head of the fish. It will be found that a current of water enters the mouth at each of these biting movements and passes out through two slits found on each side of the head of the fish. Investigation shows us that under the broad, flat plate, or _operculum_, forming each side of the head, lie several long, feathery, red structures, the _gills_.
[Illustration: Diagram of the gills of a fish. (_H_), the heart which forces the blood into the tubes (_V_), which run out into the gill filaments. A gill bar (_G_) supports each gill. The blood after exchanging its carbon dioxide for oxygen is sent out to the cells of the body through the artery (_A_).]
Gills.--If we examine the gills of any large fish, we find that a single gill is held in place by a bony arch, made of several pieces of bone which are hinged in such a way as to give great flexibility to the gill arch, as the support is called. Covering the bony framework, and extending from it, are numerous delicate filaments covered with a very thin membrane or skin.
Into each of these filaments pass two blood vessels; in one blood flows downward and in the other upward. Blood reaches the gills and is carried away from these organs by means of two large vessels which pass along the bony arch previously mentioned. In the gill filament the blood comes into contact with the free oxygen of the water bathing the gills. An exchange of gases through the walls of the gill filaments results in the loss of carbon dioxide and a gain of oxygen by the blood. The blood carries oxygen to the cells of the body and (as work is done by the cells as a result of the oxidation of food) brings carbon dioxide back to the gills.
Gill Rakers.--If we open wide the mouth of any large fish and look inward, we find that the mouth cavity leads to a funnel-like opening, the gullet.
On each side of the gullet we can see the gill arches, guarded on the inner side by a series of sharp-pointed structures, the _gill rakers_. In some fishes in which the teeth are not well developed, there seems to be a greater development of the gill rakers, which in this case are used to strain out small organisms from the water which passes over the gills. Many fishes make such use of the gill rakers. Such are the shad and menhaden, which feed almost entirely on _plankton_, a name given to the small plants and animals found by millions in the water.
Digestive System.--The gullet leads directly into a baglike stomach. There are no salivary glands in the fishes. There is, however, a large liver, which appears to be used as a digestive gland. This organ, because of the oil it contains, is in some fishes, as the cod, of considerable economic importance. Many fishes have outgrowths like a series of pockets from the intestine. These structures, called the _pyloric caeca_, are believed to secrete a digestive fluid. The intestine ends at the vent, which is usually located on the under side of the fish, immediately in front of the anal fin.
[Illustration: A fish opened to show _H_, the heart; _G_, the gills; _L_, the liver; _S_, the stomach; _I_, the intestine; _O_, the ovary; _K_, the kidney, and _B_, the air bladder.]
Swim Bladder.--An organ of unusual significance, called the _swim bladder_, occupies the region just dorsal to the food tube. In young fishes of many species this is connected by a tube with the anterior end of the digestive tract. In some forms this tube persists throughout life, but in other fishes it becomes closed, a thin, fibrous cord taking its place. The swim bladder aids in giving the fish nearly the same weight as the water it displaces, thus buoying it up. The walls of the organ are richly supplied with blood vessels, and it thus undoubtedly serves as an organ for supplying oxygen to the blood when all other sources fail. In some fishes (the _dipnoi_, page 187) it has come to be used as a lung.
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.
[Illustration: Development of a trout. 1, the embryo within the egg; 2, the young fish just hatched with the yoke sac still attached; 3, the young fish.]
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 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.
[Illustration: Artificial fertilization of fish eggs.]
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.
[Illustration: Early development of salmon. Natural size.]
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.
[Illustration: This diagram shows how the frog uses its tongue to catch insects.]
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.
[Illustration: Internal organs of a frog: M, mouth; T, tongue; Lu, lungs; H, heart; St, stomach; I, small intestine; L, liver; G, gall bladder; P, pancreas; C, cloaca; B, urinary bladder; S, spleen; K, kidney; Od, oviduct; O, ovary; Br, brain; Sc, spinal cord; Ba, back bone.]
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.