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UNIT
2
The nutrition function
DIGESTION IN
DIFFERENT ANIMALS
Natural Science 2. Secondary Education
UNIT
2
Digestion in different animals
DIGESTION
ANIMALS WITH
NO DIGESTIVE
SYSTEM
ANIMALS WITH A
DIGESTIVE
SYSTEM
WITH GASTROVASCULAR
CAVITIES
WITH DIGESTIVE
TRACTS
like
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BIRDS
Natural Science 2. Secondary Education
SNAILS
RUMINANTS
UNIT
2
Digestion in different animals
What is digestion?
• Digestion is the process by which animals transform food
into smaller components to obtain nutrients from it.
Natural Science
Science.2.
Secondary
Secondary
Education,
EducationYear 2
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UNIT
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Digestion in different animals
Animals with no digestive system
• Sponges do not have a digestive system.
• They absorb water through their pores.
• Inside their bodies, specialised cells catch the food particles in
the water, and digest them in the cytoplasm.
A cell specialised to
catch nutrients from
the water
How water
moves through
a sponge
Natural Science 2. Secondary Education
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UNIT
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Digestion in different animals
Animals with digestive systems
• In almost all animals (with the exception of sponges),
digestion is carried out by a digestive system.
• It is separated into three stages:
• Food is crushed by mechanical means. The molecules of
nutrients are broken down through chemical processes.
• Nutrients are absorbed and carried to the cells.
• The materials that cannot be used are eliminated.
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UNIT
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Digestion in different animals
Animals with gastrovascular cavities
• Cnidarians and platyhelminthes.
• Their gastrovascular cavities are bag-shaped and are
sometimes branched.
• They have only one opening, which is used both to ingest
food and to eliminate waste.
• Digestion takes place inside the cavity. The cells lining the
cavity walls absorb nutrients.
Gastrovascular
cavity
Opening
The gastrovascular
cavity of a jellyfish
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UNIT
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Digestion in different animals
Animals with digestive tracts
• A digestive tract is a long tube with an opening for food to go
in (mouth) and another opening for indigestible material to go
out (anus).
• The simplest digestive tracts, like those found in annelids,
have parts that specialise in grinding food, but they do not
have digestive glands.
Mouth
Gizzard (this grinds up food)
Anus
The digestive tract
of an annelid
Natural Science 2. Secondary Education
UNIT
2
Digestion in different animals
Animals with digestive tracts
• The most complex digestive tracts, like those found in
molluscs, arthropods and vertebrates, have specialised parts
and digestive glands that produce substances which break
down the molecules of nutrients.
Small
intestine
Oesophagus
Large
intestine
The digestive tract
of a ruminant
Ru
Mouth
Re
A
O
Stomach:
Ru: rumen
Re: reticulum
O: omasum
A: abomasum
Natural Science 2. Secondary Education
Anus
Digestive
glands:
Pancreas
Liver
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UNIT
2
Digestion in different animals
Digestion in a snail
1)
Digestion begins with the snail ingesting food with a sort of toothed
tongue called the radula.
2)
Food then passes to the mouth, where it mixes with saliva.
3)
The short oesophagus transports the food to the stomach. Although the
salivary glands surround the stomach, the saliva is released in the mouth.
4)
From the stomach, food passes to the spiral-shaped intestine, and is
expelled through the anus which is surprisingly close to the snail’s head.
Stomach
Intestine
Salivary
glands
Anus
Mouth
Radula
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UNIT
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Digestion in different animals
Digestion in a ruminant
1)
The animal ingests food without chewing and it passes to the rumen.
2)
Inside the rumen, microorganisms begin to digest the food.
3)
Parts of the food get sent back to the animal’s mouth to be chewed. This
is calling chewing cud, or ruminating.
4)
The chewed food is swallowed again, but this time it passes to the
reticulum, the omasum and the abomasum, where digestion is
completed.
5)
The food passes to the intestine, where nutrients are absorbed and
faeces is formed.
1
3
4
5
2
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UNIT
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Digestion in different animals
Digestion in a bird
1)
Food passes from the mouth to the oesophagus, where there is a pouch
called a crop. This is where food is moistened and softened to facilitate
digestion. Sometimes birds regurgitate this softened food for their young.
2)
Food then passes to the stomach, which is made up of a proventriculus,
which separates digestive fluids, and a gizzard, which has a very
muscular wall to finish grinding the food.
3)
From there, the food goes to the intestine, where it is mixed with fluids in
the liver and the pancreas.
4)
The intestine ends in the cloaca.
Proventriculus
Crop
Gizzard
Cloaca
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