Organ Systems in Organisms
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Respiration
As you should remember, humans take in oxygen through their lungs. The oxygen is
absorbed into the blood in the air sacs. Hemoglobin in the blood carries the oxygen to
all the cells of the body. Other organisms, however, have developed other ways of
getting oxygen into the cells.
Respiration in Hydras
The simplest method of getting oxygen to every cell is to place every cell in direct
contact with the source of oxygen. The source of oxygen for some organisms is oxygen
that is dissolved in water. A hydra is a very simple multicellular animal that lives in
water. (See Figure 14-2.) The hydra’s body is only two cells thick. Every cell is in contact
with the water. Oxygen that is dissolved in the water passes directly through the cell
membrane, and into each cell.
Figure 14-2. Oxygen passes from the watery environment directly into every cell of the hydra’s body
Respiration in Earthworms
How would you describe the skin of an earthworm? Slimy? Wet? The skin of an
earthworm must always be moist because the earthworm “breathes” through its skin.
Oxygen passes from the air, through the moist skin, and into the blood. Blood carries
oxygen to all of the cells of the earthworm. Why do earthworms come out of the
ground after a heavy rain? The rain replaces some of the air in the soil. There is not
enough oxygen in the rainwater to keep the earthworm alive. The earthworm must be
Organ Systems in Organisms
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careful, though. If it stays on the surface too long, its skin can dry out, and the
earthworm will die.
Respiration in Insects
Insects take in oxygen from the air, but their circulatory system does not bring oxygen
to their cells. Insect blood does not contain hemoglobin, and, therefore, cannot carry
oxygen. This is also why insect blood is not red. The circulatory system of an insect is
an open system; the blood leaves the blood leaves the blood vessels and bathes each
cell with nutrients – not oxygen. (If you see red blood when you swat a mosquito, it is
probably your blood.)
Insects take in air through tiny openings called spiracles. (See Figure 14-3.) The
spiracles branch out into small tubes that connect directly with all of the cells of the
insect’s body. The inside of these tubes are kept moist so that oxygen can be absorbed
into the cells.
Figure 14-3. A grasshopper is an insect that “breathes” through spiracles
Respiration in Fish
Like humans, fish have a closed circulatory system. The blood is contained in blood
vessels that transport oxygen to each cell. Since fish live in water, however, they do not
have lungs. Lungs are designed to absorb oxygen from air. Fish have gills that are
designed to absorb oxygen from water. (See Figure 14-4.) Gills are made of many folds
of tissue filled with blood. Water passes through the mouth of the fish and through the
gills, where oxygen is absorbed into the blood. The water then moves out through the
gill slits behind the head of the fish.
Organ Systems in Organisms
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Figure 14-4. Gills absorb dissolved oxygen from the water.
Nutrition
Humans take in food and break it down in the mouth, stomach, and small intestine. The
nutrients are absorbed into the blood in the small intestine. Water is absorbed in the
large intestine. Undigested food continues out of the large intestine. Other
multicellular organisms have developed other ways of providing nutrients to their cells.
Nutrition in Hydras
Look back at the illustration of a hydra in Figure 14-2. Each of the tentacles of the hydra
contains stinging cells, which can paralyze tiny organisms that brush against it. Once it
paralyzes an organism, the hydra uses its tentacles to draw the prey into its mouth.
Enzymes digest the food in the stomach cavity of the hydra. Any material that is not
digested is pushed back out through the mouth.
Digestion in Earthworms
Earthworms have segmented bodies. This means that their bodies consist of many
repeating rings. (See Figure 14-5.) The digestive tract of the worm is a tube that runs
through all of these segments.
Unlike the hydra’s “two-way digestive system,” the earthworm has a one-way digestive
system. Good enters the earthworm at one end, and undigested material leaves at the
other end. Earthworms live in the soil, which provides them nutrients. They take in soil
through their mouths. Muscular walls push the soil through the esophagus into the
crop, where it is stored. The soil is then pushed to the gizzard, an organ that grinds
Organ Systems in Organisms
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material into smaller pieces. This ground-up soil then moves through to the intestine,
where the nutrients are absorbed into the blood. The materials that cannot be
absorbed leave the earthworms’ body through the opening at the other end of the
earthworm. Gardeners and farmers love earthworms because they improve the soil. As
an earthworm eats its way through the soil, it creates spaces in the soil. These spaces
allow air and water to reach the roots of the plants. Soil that has many earthworms is
usually good for plants. A one-acre plot of good farmland can contain as many as 1
million earthworms.
Figure 14-5. The internal organs of an earthworm.
Organ Systems in Organisms
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Insects
What do insects eat? It depends on the insect. Since there are over 1 million different
species of insects, there are many different insect diets. Some insects eat plants, while
others such nectar from flowers. Insects may also eat wood, wool, animal wastes, blood,
small vertebrates, or even each other. The mouth of the insect is adapted to its diet.
Some contain biting parts for chewing plants or for attacking animals. Other insects,
such as butterflies, bees, and the mega-nosed fly suck nectar through a hollow tube
called a proboscis. (See Figure 14-6.)
Figure 14-6. The butterfly and the mega-nosed fly both have a long proboscis for drinking nectar.
Digestion in Insects
As you can see in Figure 14-7, the digestive system of an insect is similar to the digestive
system of an earthworm. Like earthworms, insects have a one-way digestive system.
Food passes from the mouth into a crop, where it is stored and partially digested. Once
the food leaves the crop, it enters the gizzard. The gizzard has muscular walls, and
crushes the food down to a size that can be absorbed into the insect’s body. Digestion
continues in the stomach. From the stomach, the nutrients enter the blood of the
insect, and the blood brings it to the cells. Undigested food passes into an intestine,
and then out through the end of the body.
Figure 14-7. The digestive system of the
grasshopper.
Organ Systems in Organisms
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Digestion in Vertebrates
Fish, amphibians, reptiles, and mammals are vertebrates – animals with backbones.
These animals are the most similar to humans. Vertebrates have digestive and excretory
systems that are similar to ours. They all have small intestines that deliver nutrients to
the blood.
Just as in insects, however, the mouths of vertebrates are specially adapted to the food
sources of each animal. Compare the beaks of the two birds shown in Figure 14-8. Both
the cardinal and the robin are common birds found in the Eastern United States. The
cardinal has a short conical (cone-shaped) beak, which it uses to crack open seeds and
nuts. The robin has a longer beak that enables it to dig into the ground to pull out
earthworms and insect larva.
American Robin
Cardinal
Figure 14-8. Compare the beaks of the American robin and the cardinal
Mammals, such as humans, zebras, and tigers, have teeth with which they grind down
their food. The shape of the teeth of a mammal indicates the type of food it eats.
Tigers are meat eaters. They have large, sharp teeth that are used to tear flesh. Zebras
have flat teeth that are used to grind down leaves and grasses. Humans have both
sharp teeth and flat teeth, enabling them to eat both plants and animals.
Organ Systems in Organisms
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Birds do not have teeth with which to grind their food. Instead, as in earthworms and
insects, physical digestion is carried out by the muscular walls of the gizzard. (See
Figure 14-9.)
Figure 14-9. The digestive system of a bird.
Organ Systems in Organisms
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Review Questions
Use the information below to answer questions 1-3.
All animals take in oxygen to survive. Fish absorb oxygen through their gills. In fish,
hemoglobin, the red pigment in blood, carries oxygen away from the gills to where it is
needed. Insects do not have red blood because their blood does not contain
hemoglobin and therefore cannot carry oxygen. Insects take in air through tiny
openings called spiracles. The air moves through spaces to each cell. A hydra is only
two cells thick. It does not have a respiratory system to take in oxygen. Dissolved
oxygen can move directly from the water to all of the cells of hydra.
1. The paragraph describes some of the diversity among different kinds of
organisms. Which statement best illustrates this diversity?
a. All organisms use the same structures to carry out the same life functions.
b. All organisms use the same methods to carry out the same life functions.
c. Organisms use different structures but the same methods to carry out the
same life functions.
d. Organisms use different structures and different methods to carry out the
same life functions.
2. Within the animal kingdom there are many different ways of obtaining oxygen.
Which process must occur in all animals?
a. Air is absorbed by lungs.
b. Oxygen passes into the cells.
c. Oxygen is absorbed through gills.
d. Oxygen enters blood through skin.
3. Which human organ serves the same purpose as spiracles in the insect?
a. the lungs, which absorb oxygen
b. the heart, which circulates blood
c. the stomach, which breaks down food
d. the kidneys, which eliminate liquid waste
Organ Systems in Organisms
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4. Humans, grasshoppers, birds, fish, earthworms, and many other organisms have a
one-way digestive system. Which pairing describes the organism with its oneway digestive system?
a. the grasshopper – oxygen enters through spiracles
b. the hydra – food enters and leaves through the same opening
c. the earthworm – food enters one end and undigested food leaves the
other
d. the maple tree – food is manufactured through photosynthesis
5. What is true of all animals?
a. Water is released through the skin
b. Excretion is controlled by the kidneys
c. Wastes are returned to the environment
d. Air is absorbed into the blood by the lungs
6. Explain how respiration in fish is different from respiration in humans. (2 points)
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7. Describe one similarity between the digestive systems of an earthworm and a
human, and one similarity between the digestive systems of a grasshopper and a
human. (2 points)
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