Animal Body
Systems
Section 27.2
Important Functions:
Digestion
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Sponges digest their food inside their cells 
the food cannot be larger than the cell
All other animals digest food outside their cells –
extracellular digestion  food can be larger
Simple animals have a digestive system with
one opening – gastrovascular cavity
Complex animals have a digestive tract with 2
openings – mouth and anus
“Tube within a tube” plan – allows for
specialization
Important Functions, con’t:
Respiration
In simple animals, O2 & CO2 are
exchanged by diffusion across moist skin
 Aquatic animals use gills – rich in blood
vessels, provide a large surface area,
must be moist to work
 Lungs function on land – do not require as
much moisture
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Important Functions, con’t:
Circulation
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Larger animals must have some way of
transporting O2 and nutrients
Two types of circulatory systems:
circulatory system – heart pumps blood
through the open spaces of the body cavity; not very
efficient
 Closed circulatory system – heart pumps blood
through blood vessels; blood is not in direct contact
with cells – materials diffuse through vessel walls
 Open
Important Functions, con’t:
Conduction of nerve impulses
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Needed to coordinate activities
All animals except sponges have nerves
Simple animals have a nerve net – all cells are similar
and there is little coordination
Bilaterally symmetrical animals have clusters of nerves –
ganglia
In simple animals, ganglia at the anterior end form brainlike structures
Cephalized animals – with heads – have complex
brains and sensory organs
Important Functions, con’t:
Support
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Skeleton provides support and a way for the
animal to move
Three types of skeletons:
Hydrostatic – water under pressure inside the
body cavity; found in soft-bodied invertebrates
Exoskeleton – a rigid external skeleton; found
in some invertebrates
Endoskeleton – hard material, such as
cartilage or bone, inside the animal
Important Functions, con’t:
Excretion
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The removal of wastes made by cells
These wastes are toxic – poisonous
Aquatic animals can dilute wastes with water
Land animals need to save water, so they may
produce urea, which is less toxic
Kidneys then filter urea from the blood and
produce urine
Important Functions, con’t:
Reproduction
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Asexual reproduction – only 1 cell
Parthenogenesis – new individual from an unfertilized
egg; some insects & fish
Sexual reproduction – 2 cells that join
Hermaphrodites – one animal produces both sex cells;
usually does not self-fertilize; can mate with any other
External fertilization – the egg is fertilized outside the
female; must be laid in water to keep moist
Internal fertilization – egg is fertilized inside the female
& then may given a shell & laid on land or develop inside
the mother