The Body.--One of our common fresh-water fish is the bream, or golden
shiner. The body of the bream runs insensibly into the head, the neck being
absent. The long, narrow body with its smooth surface fits the fish
admirably for its life in the water. Certain cells in the skin secrete
mucus or slime, another adaptation. The position of the scales, overlapping
in a backward direction, is yet another adaptation which aids in passing
through the water. Its color, olive above and bright silver and gold below,
is protective. Can you see how?
[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 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.
[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.