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Internal
Anatomy
Skeleton and muscles
The skeleton
of fish
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The
skeletons of most fish consist mainly of (1) skull, (2) a backbone,
(3) ribs, (4) fin rays, and (5) supports for fin rays or fins.
The skeleton of a yellow perch is shown below.
A fish's skeleton provides a framework for the head, trunk,
tail, and fins. The central framework for the trunk and tail is
the backbone. It consists of many separate segments of bone or
cartilage called vertebrae. In bony fish, each vertebra has a
spine at the top, and each tail vertebra also has a spine at the
bottom. Ribs are attached to the vertebrae. The skull consists
chiefly of the brain case and supports for the mouth and gills.
The pectoral fins of most fish are attached to the back of the
skull by a structure called a pectoral girdle. The pelvic fins
are supported by a structure called a pelvic girdle, which is
attached to the pectoral girdle or supported by muscular tissue
in the abdomen. The dorsal fins are supported by structures of
bone or cartilage, which are rooted in tissue above the backbone.
The caudal fin is supported by the tail and the anal fin by structures
of bone or cartilage below the backbone.
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all vertebrates, fish have three kinds of muscles: (1) skeletal
muscles, (2) smooth muscles, and (3) heart muscles. Fish use their
skeletal muscles to move their bones and fins. A fish's flesh consists
almost entirely of skeletal muscles. They are arranged one behind
the other in broad vertical bands called myomeres. The myomeres
can easily be seen in a skinned fish. Each myomere is controlled
by a separate nerve. As a result, a fish can bend the front part
of its body in one direction while bending its tail in the opposite
direction. Most fish make such movements with their bodies to swim.
A fish's smooth muscles and heart muscles work automatically. The
smooth muscles are responsible for operating such internal organs
as the stomach and intestines. Heart muscles form and operate the
heart. |
Systems
of the body
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The
internal organs of fish, like those of other vertebrates, are grouped
into various systems according to the function they serve. The major
systems include the respiratory, digestive, circulatory, nervous,
and reproductive systems. Some of these systems resemble those of
other vertebrates, but others differ in many ways. |
Respiratory
system
Unlike
land animals, almost all fish get their oxygen from water. Water
contains a certain amount of dissolved oxygen. To get oxygen, fish
gulp water through the mouth and pump it over the gills. Most fish
have four pairs of gills enclosed in a gill chamber on each side
of the head. Each gill consists of two rows of fleshy filaments
attached to a gill arch.
Water passes into the gill chambers through gill slits. A flap of
bone called a gill cover protects the gills of bony fish. Sharks
and rays do not have gill covers. Their gill slits form visible
openings on the outside of the body.
In a bony fish, the breathing process begins when the gill
covers close and the mouth opens. At the same time, the walls of
the mouth expand outward, drawing water into the mouth. The walls
of the mouth then move inward, the mouth closes, and the gill covers
open. This action forces the water from the mouth into the gill
chambers. In each chamber, the water passes over the gill filaments.
They absorb oxygen from the water and replace it with carbon dioxide
formed during the breathing process. The water then passes out through
the gill openings, and the process is repeated. |
Digestive
system
Digestive system, or digestive tract, changes food into materials
that nourish the body cells. It eliminates materials that are not
used. In fish, this system leads from the mouth to the anus, an
opening in front of the anal fin. Most fish have a jawed mouth with
a tongue and teeth. A fish cannot move its tongue. Most fish have
their teeth rooted in the jaws. They use their teeth to seize prey
or to tear off pieces of their victim's flesh. Some of them also
have teeth on the roof of the mouth or on the tongue. Most fish
also have teeth in the pharynx, a short tube behind the mouth. They
use these teeth to crush or grind food.
In all fish, food passes through the pharynx on the way to
the esophagus, another tubelike organ. A fish's esophagus expands
easily, which allows the fish to swallow its food whole. From the
esophagus, food passes into the stomach, where it is partly digested.
Some fish have their esophagus or stomach enlarged into a gizzard.
The gizzard grinds food into small pieces before it passes into
the intestines. The digestive process is completed in the intestines.
The digested food enters the blood stream. Waste products and undigested
food pass out through the anus. |
Circulatory
system
Circulatory system distributes blood to all parts of the
body. It includes the heart and blood vessels. A fish's heart consists
of two main chambers - the atrium and the ventricle. The blood flows
through veins to the atrium. It then passes to the ventricle. Muscles
in the ventricle pump the blood through arteries to the gills, where
the blood receives oxygen and gives off carbon dioxide. Arteries
then carry the blood throughout the body. The blood carries food
from the intestines and oxygen from the gills to the body cells.
It also carries away waste products from the cells. A fish's kidneys
remove the waste products from the blood, which returns to the heart
through the veins. |
Nervous
system
Nervous system of fish, like that of other vertebrates, consists
of a spinal cord, brain, and nerves. However, a fish's nervous system
is not so complex as that of mammals and other higher vertebrates.
The spinal cord, which consists of soft nerve tissue, runs from
the brain through the backbone. The brain is an enlargement of the
spinal cord and is enclosed in the skull. The nerves extend from
the brain and spinal cord to every part of the body. Some nerves,
called sensory nerves, carry messages from the sense organs to the
spinal cord and brain. Other nerves, called motor nerves, carry
messages from the brain and spinal cord to the muscles. A fish can
consciously control its skeletal muscles. But it has no conscious
control over the smooth muscles and heart muscles. These muscles
work automatically. |
Reproductive
system
As in all vertebrates, the re-productive organs of fish are
testes in males and ovaries in females. The testes produce male
sex cells, or sperm. The sperm is contained in a fluid called milt
The ovaries produce female sex cells, or eggs. Fish eggs are also
called roe or spawn. Most fish release their sex cells into the
water through an opening near the anus. The males of some species
have special structures for transferring sperm directly into the
females. Male sharks, for example, have such a structure, called
a clasper, on each pelvic fin. The claspers are used to insert sperm
into the female's body. |
Special
organs
Most bony fish have a swim bladder below the backbone. This
baglike organ is also called an air bladder. In most fish, the swim
bladder provides buoyancy, which enables the fish to remain at a
particular depth in the water. In lungfish and a few other fish,
the swim bladder serves as an air-breathing lung. Still other fish,
including many catfish, use their swim bladders to produce sounds
as well as to provide buoyancy. Some species communicate by means
of such sounds.
A fish would sink to the bottom if it did not have a way
of keeping buoyant. Most fish gain buoyancy by inflating their swim
bladder with gases produced by their blood. But water pressure increases
with depth. As a fish swims deeper, the increased water pressure
makes its swim bladder smaller and so reduces the fish's buoyancy.
The amount of gas in the bladder must be in-creased so that the
bladder remains large enough to maintain buoyancy. A fish's nervous
system automatically regulates the amount of gas in the bladder
so that it is kept properly filled. Sharks and rays do not have
a swim bladder. To keep buoyant, these fish must swim constantly.
When they rest, they stop swimming and so sink toward the bottom.
Many bottom-dwelling bony fish also lack a swim bladder.
Many fish have organs that produce light or electricity.
But these organs are simply adaptations of structures found in all
or most fish. For example, many deep-sea fish have light-producing
organs developed from parts of their skin or digestive tract. Some
species use these organs to attract prey or possibly to communicate
with others of their species. Various other fish have electricity-producing
organs developed from muscles in their eyes, gills, or trunk. Some
species use these organs to stun or kill enemies or prey. |
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