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INTESTINES
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Hemichordate intestines possess goblet cells
and microvilli forming a brush border extending
from tall epithelial cells (Benito, form Harrison 1997, p. 88). While the intestinal wall of urochordates is typically smooth, a few species possess grooves.
Intestinal cells include absorptive cells, endocrine cells (whose
secretions include gastrin, pancreatic polypeptide,
and secretin), and mucus cells. There is very
little muscle along the GI tract and most material is moved through ciliary
action (Burighel, from Harrison, 1997, p. 256)
In lancelets, cells of the stomach and intestine are ciliated.
Endocrine cells exist in the intestine and a muscular sphincter surrounds
the anus (Ruppert, from Harrison, 1997, p. 444-5;
Burighel, from Harrison, 1997, p. 255). In lancelets, cilia move food to the intestine
where flagella mix with digestive enzymes produced primarily from the
hepatic cecum (Ruppert, from Harrison, 1997,
p. 354). Unlike hemichordates and protostomes, the digestive tract of chordates does not proceed
to the tip of the abdomen. One
of the major traits which all chordates share is that of a muscular, postanal tail. |
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LANCELET |
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LAMPREY |
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ZEBRAFISH EMBRYO |
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Fish increase the surface area of the intestine
through a fold which runs along the intestine (the typosole in jawless fish and the spiral valve in cartilaginous
and primitive bony fish). Placoderms also possessed a spiral valve in the intestine.
(Weichert, 1970, p. 187) Some
teleosts have increased the length of their intestines while
others possess many blind pockets near the proximal end of the intestine
called pyloric caeca (mackerel may have 200
of these structures) (Webster, 1974).
Most fish lack intestinal glands which extend into the submucosa
(Stevens). Tetrapods increase the surface area
of their small intestine through finger-like projections known as villi. Tetrapods possess lymphatic capillaries known as lacteals
in these villi and possess glandular crypts
of Lieberkuhn in their intestines. (Stevens,
p.18). The villi of a frog’s intestine is
pictured below. |
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In amphibians, there is an ileocecal valve separating the small and large intestines
(Romer, p. 388) and the proximal portion of the small intestine
becomes the specialized duodenum (Kardong, p.
512). Tetrapods
lose the intestinal spiral valve which is present in gnathostome
fish (Romer, p. 387).
In about 2% of humans, a portion of the yolk stalk is retained
as the Meckel’s diverticulum from the ileum.
Although this pouch is usually about 2 inches, it can reach 7 inches
(and may even open through the navel, requiring surgery). (Weichert,
1970, p. 189). Human intestines can vary between 11 to almost 26
feet in length (Weichert, 1970, p. 186). Intestinal sodium transport similar to that
which occurs in vertebrates is known in most coelomates
(Stevens, p. 296). In mammals, intestinal villi
become more prominent (Romer, p. 386). Higher
apes developed an appendix (Ankel-Simons, p.
385). Some edentates possess 2 ceca
(Weichert, 1970, p. 189) Some insectivores
and one rodent (the mountain beaver Aplodontia rufa) possess a cloaca (Stevens).
In amniotes,
a blind pouch known as the cecum exists where
the small intestine joins the large intestine (Romer,
p. 388). There are a number of
variations of the cecum known in mammals. The cecum varies in
the armadillo genus Dasypus
from being absent to the presence of two small ceca
(Stevens). In viverrids
(carnivores), the cecum may be present, vestigial,
or absent (Stevens, p. 58). Many
marsupials lack a cecum (Stevens). |
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HAGFISH |
LAMPREY LARVA |
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LAMPREY |
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SHARK |
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BOWFIN |
PERCH |
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LUNGFISH |
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SALAMANDER |
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FROG |
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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 |
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GOAT |
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COW |
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PIG
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MONKEY |
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HUMAN MODEL |
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