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GASTROINTESTINAL TRACT
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ORGANS OF THE GASTROINTESTINAL TRACT There are similarities of the gastrointestinal
tract shared by chordates. Food
enters the body through the oral cavity (mouth), as in that of a lancelet
pictured below. |
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PHARYNX From the mouth, food passes through the pharynx
to the intestine. The ability
to pass water through pharyngeal slits is involved in feeding in primitive
filter-feeding chordates and hemichordates: food from the water becomes
stuck in the mucus lining the pharynx and is carried to the intestine
by ciliary action. Pharyngeal
slits became involved with respiration secondarily and higher vertebrates
use this mucociliary mechanism as a way of directing
microbes and dirt to the stomach and away from the lungs. Most filter
feeding animals make use of mucus to capture the food and cilia to transport
it. The development of a perforated,
muscular pharynx in chordates allowed the change from microphagy (feeding on small material suspended in water)
to macrophagy (feeding on larger material)( |
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ESOPHAGUS In hemichordates, food travels from the pharynx
into the esophagus where peristalsis moves it through the remainder of
the gastrointestinal tract (Burighel, from Harrison,
1997, p. 255) |
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HAGFISH |
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SHARK |
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PERCH |
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The esophagus became more prominent in tetrapods than in fish (Romer, p.
378). The primitive condition of
esophageal cilia still persists in some fish, amphibians, and reptiles
(Stevens; Weichert, 1970, p. 178). Reptiles have a much longer esophagus in contrast
to the short organs observed in fish and amphibians. Reptiles are the first group in which there
is a clear distinction between the stomach and esophagus (Weichert,
1970, p. 181). In birds the esophagus
forms a separate sac known as the crop.
A few birds energy rich secretions from
the crop known as “crop milk” to nourish hatchlings (Webster, 1974). The muscle of the esophagus in fish is striated
in contrast to the majority of tetrapods in
which the esophagus is lined by smooth muscle.
The amount of smooth and striated muscle found in the esophagus
varies in different groups of mammals (Stevens). Ruminants possess skeletal
muscle down their entire esophagus, giving them a greater voluntary control
over its action (as evidenced in their ability to regurgitate swallowed
food, the cud, to continue chewing it).
In some fish, amphibians, and a bat (Plecotus auritus) the
esophogus is lined with columnar epithelia
which secretes pepsin and/or HCl. Sometimes this condition is even observed in
humans (Stevens). The crossing
of the passageways for air and food makes an animal vulnerable to asphyxiation.
In whales, there is no pharyngeal chiasma since
the larynx wraps around the pharynx reach the nostrils (Webster, 1974). |
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PERCH |
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FROG |
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ALLIGATOR |
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CHICKEN![]() |
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OPOSSUM |
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CAT |
PIG |
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SHEEP![]() |
GOAT |
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MONKEY |
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STOMACH In filter feeders, food-bearing mucus is
transported to an acidic area of the gastrointestinal tract where the
mucus becomes less viscous and the food is released (Hoar, 1983, p. 420). Stomachs evolved in the ancestors of gnathostomes. In jawless
fish, the stomach is virtually nonexistent, represented by a small enlargement
of the distal end of the esophagus. (Weichert, 1970, p. 180). In gnathostomes (and
perhaps late fossil jawless fish such as thelodonts),
part of the GI tract develops into a stomach. |
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In gnathostomes, the
stomach can be divided into several regions. There is a body where the food is mechanically
and chemically digested, a fundus where food
can be temporarily stored, and a narrow pyloris
region which is separated from the small intestine by the pyloric sphincter.
The glandular cardiac region of the stomach is
shared by tetrapods (Stevens, p.16). Mammals have separate cells for the secretion
of pepsin and hydrochloric acid while these tasks are performed by the
same cells in other vertebrates (Webster, 1974, p. 345). In the two monotremes,
the stomach is lined with stratified squamous
epithelia (Stevens). Some insectivores possess a stomach lined by keratinized
epithelium to protect them from hard insect skeletons and release their
gastric secretions from one duct rather then through numerous gastric
pits. Ruminants possess a four chambered stomach which
contains the symbiotic microbes which help them digest plant material
and can hold up to 60 gallons of material (Webster, 1974). The stomachs of ruminant artiodactyls, whales,
and hippos are also divided into compartments (Weichert,
1970, p. 183). There is considerable
stomach variation in bats; in vampire bats the stomach forms a long tube. The bat Desmodus possesses a pouch at the
distal end of its stomach which holds the ingested blood (Weichert,
1970, p. 183). Kangaroos also possess
a long, tubular stomach (Stevens). |
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SHARK |
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GAR |
BOWFIN![]() |
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FROG |
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SALAMANDER |
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TURTLE |
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ALLIGATOR |
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CHICKEN |
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OPOSSUM |
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CAT |
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SHEEP |
GOAT |
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COW |
PIG |
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MONKEY |
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HUMAN MODEL |
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