C HAPTER 27: I NTRODUCTION TO A NIMALS Section 2: Animal Body
Systems
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T ISSUES AND O RGANS Digestion Single celled organisms and
sponges digest their food within their body cells. All other
animals digest their food extracellularity (outside of their body
cells) within a digestive cavity.
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T ISSUES AND O RGANS Simple animals, such as the hydra and
flatworms, have a gastrovascular cavity, a digestive cavity with
only one opening.
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T ISSUES AND O RGANS Other animals have a digestive tract (gut)
with two openings, a mouth and an anus.
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T ISSUES AND O RGANS Respiration In simple animals, oxygen gas
and carbon dioxide gas are exchanged directly with the environment
by diffusion.
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T ISSUES AND O RGANS The uptake of oxygen and the release of
carbon dioxide, called respiration, can take place only across a
moist surface.
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T ISSUES AND O RGANS Some aquatic (and a few terrestrial)
animals respire with gills very thin projections of tissue that are
rich in blood vessels.
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T ISSUES AND O RGANS In more advanced animals, lungs are the
respiratory organs used to transfer oxygen into the blood and
remove carbon dioxide from blood.
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T ISSUES AND O RGANS Circulation In complex animals, oxygen and
nutrients must be transported to the body cells by a circulatory
system.
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T ISSUES AND O RGANS Two types of circulatory systems: Open
circulatory system: heart pumps fluid containing oxygen and
nutrients through a series of vessels out into the body
cavity.
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T ISSUES AND O RGANS Closed circulatory system: a heart pumps
blood through a system of blood vessels.
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T ISSUE AND O RGANS Conduction of Nerve Impulses Nerve cells
(neurons) are specialized for carrying messages in form of
electrical impulses. Bilaterally symmetric animals have clusters of
neurons called ganglia.
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T ISSUE AND O RGANS More complex invertbrates, such as the
grasshopper, have brains with sensory structures.
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T ISSUE AND O RGANS Support Many soft-bodied invertebrates have
a hydrostatic skeletal systems. Hydrostatic skeleton - consists of
water that is contained under pressure in a closed cavity.
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T ISSUE AND O RGANS Other invertebrates, such as insects, have
an exoskeleton, which is a rigid external skeleton that encases the
body of an animal.
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TISSUE AND O RGANS An endoskeleton is composed of a hard
material, such as bone, embedded within an animal.
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T ISSUE AND O RGANS Excretion The term excretion refers to the
removal of wastes produced by cellular metabolism.
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T ISSUE AND O RGANS Simple aquatic invertebrates and some
fishes excrete ammonia into the water through their skin or gills
by diffusion.
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T ISSUE AND O RGANS Other animals, especially terrestrial
animals, convert ammonia to nontoxic chemicals, like urea. As the
excretory system eliminates these wastes, water and other useful
substances are returned to the body.
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R EPRODUCTIVE S TRATEGIES Asexual Reproduction Reproduction
that does not involve the fusion of two gametes is called asexual
reproduction. An unusual method of asexual reproduction is
parthenogenesis, in which a new individual develops from an
unfertilized egg.
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R EPRODUCTIVE STRATEGIES Animals that reproduce asexually are
usually able to also reproduce sexually.
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R EPRODUCTIVE S TRATEGIES Sexual Reproduction In sexual
reproduction, a new individual is formed by union of a male and a
female gamete.
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R EPRODUCTIVE S TRATEGIES Gametes are produced in the sex
organs. Males have testes that produce sperm. Males produce sperm
until death.
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R EPRODUCTIVE S TRATEGIES Females have ovaries that produce
eggs. At birth, females have produced all the eggs they will ever
have.
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R EPRODUCTIVE S TRATEGIES Some species of animals, called
hermaphrodites, have both testes and ovaries.
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R EPRODUCTIVE S TRATEGIES Most aquatic animals simply release
the male and female gametes near one another in the water, where
fertilization occurs. This is known as external fertilization.
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R EPRODUCTIVE S TRATEGIES Most terrestrial animals sexually
reproduce by means of internal fertilization. Internal
fertilization occurs when the sperm and egg unite inside of the
females body.
