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How Animals Eat, Digest, and Absorb Food
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Covers modes of ingestion, ad how food and different molecules are digested.
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Comparative Anatomy and Physiology of AnimalsLecture 16 How do animals eat, digest and absorb food? 09/12/20
How do animals eat?
- 4 modes of ingestion: filter feeding, deposit feeding, fluid feeding, and bulk feeding.
Filter feeding
- Filter feeding is the action of obtaining nutrients from particles suspended in water.
- Water is usually passed through a specialised filter system, such as baleen or radioles.
- Animals that employ this mechanism include whale sharks, flamingos, lots of aquatic
invertebrates, and baleen whales.
Anatomy of the mouth
- Baleen whales: either swim open-mouthed and
continually filter water, or lunge at a
concentration of krill, gulp a large mouthful of
water, then squeeze the water out through
their baleen. Baleen is a collection of long, hair-
like structures that hang from the upper jaw
and are used to capture krill.
- Barnacles: small crustaceans which attach to a
surface, such as rocks or other animals, and
filter feed. They cement themselves to a surface
by her head and they have feathery structures
which protrude from the shell, and these are
their feet. These feet filter plankton from the
water and pass them to the mouth.
- Sea squirts: also known as tunicates. They feed
by taking in water
through the mouth (or the oral/incurrent siphon). From here,
the water flows through mucus-covered gill slits (AKA
pharyngeal stigmata) into a water chamber called the atrium,
and then exits through the atrial siphon. Tentacles at the
opening trap particles and then pass them to the stomach.
- Cnidarians: suck water in as a function of their motion. Catch
prey using their tentacles, which
are then passed to the mouth.
Unlike most animals, cnidarians
only have a gastrovascular cavity
and no alimentary tract. This acts as both the mouth and
anus.
Deposit feeding
- Deposit feeding is the action of obtaining nutrients from particles suspended in soil.
, - Animals that do this include earthworms, sea cucumbers, fiddler crabs, and sand dollars
(echinoderms).
Anatomy of the mouth
- Earthworm: the gut is essentially a straight tube
which extends from the worm’s mouth to its anus. It
differentiates into a buccal cavity, pharynx,
oesophagus, crop, gizzard usually, and intestine.
Food enters the mouth. The pharynx acts as a
suction pump, drawing in the food. In the pharynx,
the pharyngeal glands secrete mucus. Food then
passes into the oesophagus, where calcium is pumped in to maintain proper calcium levels
in the blood, as well as food pH. From here the food passes into the crop and gizzard, where
strong contractions of muscles grind the food, along with mineral particles ingested with the
food. After this, the food goes through the intestine for digestion.
- Benthic sea cucumbers: not all species. Are
the same as earthworms and use tentacles
to push food towards the mouth. Passed
through the intestine where nutrients are
absorbed, and all the filtered sediments
are excreted through the anus.
Fluid feeding
- Fluid feeding is the action of obtaining
nutrients by consuming other organisms’ fluids.
- Plant sap: aphids, shield bugs.
- Nectar: butterflies, bees.
- Blood: mosquitoes, assassin bugs, ticks, mites, fleas, leeches, vampire bats.
- Blood feeding is especially important for transmission of diseases
in animals.
Anatomy of the mouth
- Mosquitoes: delicate syringe-like mouthparts. The labium is like a
sword sheath housing the other parts which form a hollow needle.
This is inserted into the skin and searches
around for a capillary, then sucks up the
blood.
- Horsefly: has mandibles and
maxillae which it uses to cut into the
skin, and then uses the labium as a sponge to lap up the
blood that comes out.
Bulk feeding
- Bulk feeding is the action of obtaining nutrients by eating some or all an organism.
- The animals do this include most vertebrates, many invertebrates, and includes herbivores,
omnivores, and carnivores.
Anatomy of the mouth
- Birds: do not have teeth to masticate food, so they use
their beaks and tongues to pick up and manipulate food.
In some cases, the beak can also break up food, as seen
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