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ACCESSORY
DIGESTIVE STRUCTURES
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The
digestive system is composed of organs of the gastrointestinal tract through
which ingested food passes and a number of accessory digestive structures
whose function facilitates digestion although food never actually passes
through these structures. In humans,
the liver, pancreas, and gall bladder are accessory structures which release
secretions into the small intestines through ducts.
The mouth possesses teeth, a tongue, and salivary glands which
are needed for chewing. THE MOUTH The hemichordate enteropneusts
are mucociliary feeders. A preoral ciliary organ may be involved in the sensory recognition of
food. The mouth’s only role in
feeding is to propel the food to the pharynx and no special digestion
or manipulation occurs here. (Benito, form Harrison
1997). Although jawless fish possess tongues, the tongues
of jawless fish are not homologous to the tongues of higher vertebrates.
The tongues of gnathostome fish are supported by the hyoid bone but lack
significant muscles and can achieve only minimal movements.
Fish tongues may possess sensory receptors, papillae, or even teeth.
Some amphibians possess a tongue similar to those of fish, some toads
have lost their tongues altogether, and other amphibians possess highly
modified, mobile tongues. Perhaps the evolution of more muscular tongues
was a result of the pharynx no longer being used in respiration and the
need to moisten food in terrestrial environments.
Tongue muscles are derived from hypobranchial
muscles (from the gill arches) and anchored to the hyoid (which is composed
of the fused cartilage of several gill arches). Turtle, crocodilian, and
bird tongues are capable of little movement; those of birds lack most
intrinsic muscles (Romer, p. 328). The
mammalian tongue is the most highly developed vertebrate tongue which
is formed by 5 separate embryonic structures which fuse during development. In anteaters the tongue is very large some of
the tongue muscles actually originate on the sternum. In the tongue, a
depression known as the foramen cecum marks
the original location of the thyroid gland (Weichert,
1970, p. 164). In amniotes, the
hyoid bone which supports the tongue is located more caudally (Webster,
1974, p. 329). |
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MONKEY |
Fish
possess taste buds which are not restricted to the mouth; they can actually
cover the body. In tetrapods
they are located in the mouth and pharynx while in mammals they are concentrated
on the tongue (Romer, p. 328). Human taste buds are depicted in the following
two images. |
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GAR |
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Although
all gnathostomes have lips (Torrey,
1979, p. 317), movable
lips occur only in mammals (except in the platypus and whales) and evolved
together with facial muscles (Weichert, 1970,
p. 158). Mammals also developed cheeks and a variety
of mammals have developed additional cheek pouches (such as some rodents,
some monkeys, and the platypus) (Romer, p. 326). Modifications of the jaw and jaw musculature
allow chewing in mammals which other vertebrates are unable to perform (other
than many extinct ornithishian dinosaurs). |
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TEETH Although jawless fish and fossil conodonts have tooth-like structures in the mouth, only gnathostomes possess true teeth. |
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HAGFISH |
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The dermal denticles
in the skin of cartilaginous fish are homologous to teeth and teeth probably
developed from denticles in the mouth area in
fossil placoderms as a supplement to their sharp biting jaw bones
(Romer, p. 333-6).
Primitive vertebrates may possess teeth on the roof of the mouth
(palate) and extra rows of teeth on the bones of the jaws. These extramarginal
teeth had been reduced over the course of vertebrate evolution and mammals
lost the palatal teeth which exist even in some reptiles (Romer,
p. 328). Most sharks possess a homodont set of teeth
(all the teeth have the same shape), but a few sharks possess a heterodont dentition with sharp tearing teeth in front and
crushing teeth located posteriorly. (Webster, 1974). Actinopterygians may have homodont or heterodont
teeth or lack teeth altogether. In
many fish teeth are attached to bone by collagen fibers while in others
they are located in deep sockets (Webster, 1974). A number of dinosaurs
(such as the basal ornithopod in the following
illustration) evolved a heterodont condition
with different kinds of teeth. |
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The sarcopterygians
which evolved into amphibians possessed teeth with many grooves (giving
a labyrinthine appearance in cross section).
The first amphibians were called labyrinthodonts
because of their retention of this condition (Webster, 1974, p. 334). While the most primitive teeth were simply
located in shallow grooves in the jaws, mammalian teeth are embedded in
sockets (Kardong, p. 497). In mammals, an additional layer of teeth known
as cementum anchors the root of the teeth (Weichert, 1970, p. 169). Monotremes
possess horny egg teeth, as in reptiles, to help them escape from their
shell when they hatch. (Weichert, 1970, p. 173). Because mammalian teeth are only replaced once,
selection has resulted in occluding surfaces in the upper and lower jaws.
Occasionally in humans postpermanent teeth have been observed which allow them to
replace their “adult” teeth (Weichert, 1970,
p. 172). |
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Mammals
have heterodont teeth (different kinds): incisors,
canines, premolars, and molars. The
number of teeth has decreased over the course of mammalian and primate
evolution. The catarrhine
primates ( Jawless fish, some amphibians, sirens, and
the platypus possess epidermal teeth formed by cornified
papillae. Whales can produce more than 2 sets of teeth.
Baleen in whales is composed of cornified papillae which are joined. Rodents’ incisors continue to grow throughout
life. In the narwhal, the left
upper canine in males can grow to be 8 feet long. (Webster,
1974; Weichert, 1970, p. 160). Herbivores which feed on grass often develop
high crowned teeth in which very high cusps form with a layer of cementum over the enamel.
As the tooth wears down, it maintains its very hard ridges (Webster,
1974). Incisors have been modified
in elephants, boars, and walruses to form tusks.
The modified teeth of different fossil elephants are depicted in
the following illustrations. |
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. The only digestive glands in the mouths of fish
are simple mucuous glands. Once vertebrates adapted to terrestrial environments,
glands became necessary to moisten food before swallowing. A number of oral glands are known from modern
amphibians (including the lingual glands) and from modern reptiles (including
the lingual and sublingual glands). Mammals
develop parotid, submandibular (or submaxillary), and sublingual glands. There have been a few modifications in the ancestral
set of salivary glands in mammals: rabbits and horses lack the submandibular glands while mice and some insectivores lack
the sublingual glands. The salivary
glands in general are reduced or absent in whales and sirens. Artiodactyls
possess additional molar glands and dogs possess orbital glands (Weichert, 1970, p. 161-2). |
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PANCREAS Amphioxus
lacks a pancreas. In jawless fish,
diffuse patches of pancreatic tissue exist which empty separately into
the small intestine, similar to the multiple units of the embryonic pancreas
of higher vertebrates (Romer, p. 393-5). In gnathostomes,
the units of the pancreas unite during embryological development and the
exocrine and endocrine cells fuse to form a composite structure (Kardong, p. 519). The compact pancreas of sarcopterygians
more similar to tetrapods than actinopterygians
(Youson, 1999). Some cats possess a pancreatic bladder which
releases pancreatic secretions in a way similar to the action of the gall
bladder (Weichert, 1970, p. 197). |
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PERCH |
FROG |
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ALLIGATOR |
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OPOSSUM |
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CAT |
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MONKEY |
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HUMAN MODEL |
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