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Digestive System: Oral Cavity and
Feeding Mechanisms
Introduction
• Acquiring food for energy to drive metabolism
and other physiological processes is central to the
survival of the individual.
• Competition for food resources is often intense in
the animal’s natural surroundings, therefore,
increasing the efficiency with which you collect
and process food is at a premium in natural
selection.
• Vertebrates have evolved many different designs
with which a vast array of food resources is
exploited
Mouth and Oral Cavity
• The mouth opening and the oral cavity are variable
parts of the digestive tract because vertebrates gather
and ingest many different kinds of food in numerous
ways.
– Even the position of the mouth opening is not the same.
• The mouth of a jawless hagfish develops near the
caudal end of the stomodaeum.
– In most jawed fishes it develops at a level between the
hypophyseal sac and a pair of embryonic nasal placodes.
• In choanate fishes and tetrapods, the mouth opening is
at the level of the nasal placodes
Basic Modes of Feeding
• Oral structures are adapted to the way in which food is
gathered.
• The urochordates, cephalochordates, and ammocoetes
larva are jawless suspension-feeders.
– Some combination of ciliary currents and expansions of
the buccal cavity move water through the mouth and
pharynx.
– Suspended food particles are trapped by mucus and
carried to the esophagus.
• The evolution of teeth and jaws enabled vertebrates to
feed in other ways.
• Most fishes are suction-feeders and capture
prey by drawing water and prey into the
mouth.
– Some fishes are ram-feeders that simply overtake
prey with their mouths open.
• Many primitive terrestrial verts transport food
within the mouth by rapidly advancing the
head in relation to food.
– A process called inertial-feeding.
• Most vertebrates swallow their food whole or
in large chunks.
– Most mammals and some reptiles masticate their
food.
Teeth
• Living jawless fishes lack teeth, but keratinized
cones called teeth are associated with the mouth
and tongue of adult hagfishes and lampreys.
• True teeth evolved along with jaws and are
present in some part of the oral cavity in
gnathostomes.
– Unless they have been secondarily lost.
• A representative adult tooth is composed of
bonelike dentine covered by a hard layer of
enamel.
– Their structure is similar to bonny scales.
• In fishes teeth may be distributed throughout
the oral cavity and pharynx.
• Tetrapod teeth usually are limited to the jaw
margins and sometimes the palate.
– They attach to the underlying structure by
connective tissues that form a periodontal
ligament and also by an acellular bone; cement.
– A superficial attachment of the tooth to the jaw is
called Arcodont, or Pleurodont; and these teeth
vary only in the location of their attachment.
– Teeth may also be set within deep sockets in the
jaw, an attachment termed thecodont.
• In most vertebrates new teeth can be
generated from the tooth germ throughout
the animal’s life, such vertebrates are called
polyphyodont.
– The new tooth begins to form before the old tooth
is lost.
– Loss and replacement are not random, but follow
a complex cycle
• Most mammals are diphyodont and have 2
sets of teeth: milk and permanent.
– Toothed whales are monophyodont and have only
a single set of teeth.
• Although tooth size may vary, all teeth of most
fishes, amphibians, and reptiles have a similar
shape, a condition termed homodont.
• The shape depends on how many teeth are used.
– In most fishes and early tetrapods they are simple
cones, and serve primarily to prevent the escape of
caught prey.
– Shark teeth are often triangular, with sharp serrated
edges that are used to pierce and tear away chunks of
flesh from prey items.
• Teeth have been reduced or lost where they
serve no purpose in a species feeding method.
• The ingestion and mastication of food in
mammals has been accompanied by the
evolution of teeth specialized for different
functions.
– Such dentition with teeth of varying shape is
called heterodont.
• Primitive placental mammals have three small
incisor teeth at the rostral end of each side of
the upper and lower jaw, followed by a single
canine, 4 premolars, and 3 molars.
• The configuration of particular teeth are
adapted to their use.
• Incisors are typically small, spade-shaped
teeth used for cutting, cropping, and picking
up food.
• Canines are usually large, conical teeth used
to seize, pierce, and kill prey.
• Primitively the premolars are puncturing
teeth, and the molars have a combined
cutting and crushing action.
– These teeth, collectively called cheek teeth are the
most complex and variable among species.
Feeding Mechanisms of Vertebrates:
The Aquatic Medium
• Early in the evolution of the feeding mechanism
in ray-finned fishes is the primitive pattern seen
in “paleoniscoid” fishes in which the mouth is
opened by epaxial muscles that lift the head.
• At the same time ventral (hypaxial) body muscles
and the interhyoideus muscle, pull down the
lower jaw.
– This creates a wide gape, but no suction, and these
fishes capture prey through ram-feeding.
• Modern sharks still open their jaws with a similar
method, although some suction-feeding takes
place.
• A new skull design along with the
dermatocranium evolved in the actinopterygian
fishes.
– It is characterized by a greater number, and mobility,
of bony elements.
• During evolution in bony fishes, five major
structural changes occurred to the jaws:
1. In the upper jaw, the premaxilla becomes greatly
enlarged, highly mobile, and the only toothed
element, while the highly mobile maxilla loses its
teeth.
2. The jaw suspension, changes from oblique to a more
vertical position so that the volume of the mouth
cavity is substantially increased.
3. The jaw joint, also, was shifted from a position
posteroventral to the orbit of the eye, to a position
anteroventral to it.
4. The operculum, which was originally rigidly attached to
the skull, is now separate and articulates with the
palatopterygoid arch.
5. The hyoid becomes connected to the gill cover by a
ligament, so that movement of the hyoid is indirectly
transferred to the lover jaw.
• In this advanced design the mouth is opened by
lifting the head through the action of epaxial
muscles, while the simultaneous action of the
hypaxial and sternohyoideus muscles pulls the hyoid
back and downwards; which in turn pulls on the
mandible causing it to drop.
• In advanced teleosts, a third mechanism to
open the mouth evolved.
• It involves a rotation of the gill cover by the
action of a muscle that lifts the operculum;
this rotation is transmitted by ligament to the
posterior corner of the mandible, causing it to
drop.
• While the jaw is lowered, the premaxilla slides
forward, protruding, and at the same time the
volume of the buccal cavity increases
significantly as the palatopterygoid arch flares
out as the floor drops.
Swallowing: Aquatic Anamniotes
• Once prey has been captured it must be
transported into the pharynx and esophagus.
• Aquatic anamniotes use hydraulic transport; in
both fishes and aquatic amphibians, water
currents within the mouth transport prey.
Terrestrial Feeding: Amniotes
• One of the major features that changed as
animals moved from the aquatic to the terrestrial
environment was the feeding structures.
• Significantly different designs of feeding
apparatus are required because air is les dense
that water, so the prey or food is air cannot be
sucked in.
– The essential difference is that coordinated
movements of the mandible and tongue replace water
flow in terrestrial feeding.
• In all terrestrial vertebrates the feeding cycle had
4 phases: slow opening, fast opening, fast closing,
and slow closing or power stroke.
1. The cycle begins with slow opening of the mandible.
•
In lizards and other amniotes with kinetic skulls, the snout
lifts up in relation to the braincase.
2. This stage is followed by fast opening, during which a
sudden and rapid opening of the mouth to maximum
gape occurs.
3. After this, the fast closing mechanism is activated by
strong contractions of adductor muscles.
4. The final stage is slow closing during which the snout
is depressed by the strong action of the pterygoideus
muscle and at the same time continuous
contractions of the adductor mandibulae causes a
strong bite.
• Mastication requires a precise positioning of the
lower teeth relative to the upper ones (occlusion),
and the positions may change as food is gathered
and processed.
– Rodents and elephants chew with a fore and aft motion of
the jaw, most other mammals chew with a side to side
motion.
• A mammal usually chews on only one side of its
mouth at a time.
• Teeth, the shape and size of the jaw, the shape and
position of the jaw joint, and the size and
arrangement of the jaw muscles are adapted to the
type of food eaten.
Tongue, Cheek, and Lips
• The primary difference between aquatic and
terrestrial feeding is the mechanism of food
transport.
• In aquatic feeding, water currents created by
movements of the bones of the skull and hyoid
transport prey within the mouth and pharyngeal
cavity.
• In contrast, terrestrial verts rely on movements of
the tongue
– Tetrapods have evolved mobile, muscular tongues
that are supported by the hyoid and anterior branchial
arches.
• In both reptiles and mammals, tongue
movements are correlated with the slow and fast
openings/closings of the jaw.
• In mammals the movements of the tongue are
also correlated to movements of the mandible
and hyoid and movements produced within the
tongue itself.
– The tongue is often used in food gathering, but has
many other functions.
• Fleshy cheeks and lips characterize most
mammals and assist the tongue in manipulating
food within the mouth, and are used by
newborns to suckle.
Oral Glands
• Most fishes lack oral glands, apart from scattered
mucus-secreting cells.
– Lamprey are the exception and have a large pair of
glands that secrete an anticoagulant needed to keep
their preys blood flowing.
• Oral glands, including salivary glands are well
developed in terrestrial vertebrates.
• Their mucus and serous secretions lubricate food
and facilitate food manipulation and swallowing.
Evolution of the Vertebrate Feeding
Mechanism
• Major changes in the feeding apparatus and behavior
have emerged during vertebrate evolution.
• Five primitive features have been identified:
1. Prey transport is effected by water currents
2. The gape remains constant prior to fast opening
3. Mouth opening is produced by both pronounced lower
jaw depression and head elevation.
4. A recovery phase follows prey ingestion
5. Hyoid retraction by the sternohyoideus is coincident with
the fast-opening stage.
• These characteristics are present in all sharks and bony
fishes.
• Only feature 5 remains constant during evolution.
• In tetrapods a muscular tongue evolved for
prey transport within the oral cavity.
• This feature becomes further elaborated in
amniotes, along with 3 other features:
1. Chewing and prey transport begins with a slow
opening stage.
2. The gape is produced by lowering the mandible
rather than by head lifting.
3. The recovery phase is no longer distinguishable.
• 2 evolutionary innovations emerge in
amniotes: inertial feeding and a 4 stage
masticatory cycle of food processing.
The Digestive System: Pharynx
Stomach and Intestine
Introduction
• The structure of the digestive tract and its evolution
are affected by many factors, including the type of food
eaten, the level of activity and metabolism of the
animal, and the size of the animal
• Digestion occurs primarily in the stomach and
intestine.
• Many digestive enzymes acting in sequence are needed
to break down the food into small molecules.
– These molecules must then be absorbed along with
minerals and other nutrients.
1. Food that has been transported through the
oral cavity next enters the pharynx.
– From there it passes through the esophagus,
stomach and intestine.
2. Large quantities of water are released into
the digestive tract as a component of the
digestive secretions and must be reabsorbed.
3. Finally, the undigested residues, cells
sloughed off the digestive tract lining, and
bacteria must be eliminated, or egested.
The Pharynx and its Derivatives
• The pharynx arises from the anterior part of
the archenteron and is the part of the
digestive tract from which the paired
endodermal pharyngeal pouches arise.
– Jawed fishes have 6 pairs of pouches that extend
laterally, meet the ectodermal furrows, and open
to the surface.
• As food passes through the pharynx of a fish it is
prevented from entering the branchial chambers by the
gill rakers.
• In tetrapods the pharynx is very short, it is
little more than a connection between the
oral cavity and esophagus.
– The lungs of tetrapods develop as ventral
outgrowths from the floor of the caudal part of
the pharynx.
• Food passing through the pharynx of
terrestrial verts is prevented from entering the
respiratory passage by the closure of a slit-like
glottis in the floor of the pharynx.
– In addition, mammals have a flap-like epiglottis
that lies rostral to, and deflects food away from,
the glottis.
The Thymus and Tonsils
• The vertebrate thymus develops from the
endodermal epithelium of certain pharyngeal
pouches and from the adjacent ectodermal
epithelium.
– After separating from the pouches the epithelial
thymus receives stem cells from the spleen and
bone marrow and converts them into Tlymphocytes
• Tonsils are lymphoid organs that participate in
the tetrapod’s immune response.
Gut Tube Structure
• The post pharyngeal digestive tract is a tubular
structure, parts of which are often enlarged into
saccular chambers.
– The lining of the gut tube is a stratified squamous
epithelium in the parts exposed to constant abrasion:
oral cavity, pharynx, esophagus, and cloaca.
– It is a simple columnar epithelium in all or most of the
stomach and intestine.
– A loose fibrous connective tissue forms the rest of the
wall.
• The innermost layer, called the mucous
membrane, or mucosa, is the most complex layer.
– It consists of the simple columnar epithelium lining
the intestine and a fibrous connective tissue
containing many blood and lymphatic vessels that
receive absorbed materials.
– The surface bears numerous microvilli that greatly
increase the digestive and absorptive surface area.
– The surface area of the mucosa is also increased by
the presence of large folds
– Many of the epithelial cells, called goblet cells, release
mucus that lubricates the lining and prevents
autodigestion.
• A second layer, the submucosa lies deep to
the mucosa.
• A layer called the muscularis lies peripheral to
the submucosa.
– It is composed of layers of muscle that spiral
around the intestine and are antagonistic to each
other.
– Their activity is integrated by the enteric part of
the autonomic nervous system.
– Peristaltic contractions of the muscle layers
propels food through the digestive tract and
churns it, thereby helping break it up mechanically
and mix it with digestive secretions.
• Churning is particularly pronounced in the stomach.
The Esophagus
• The esophagus is a connecting segment between
the pharynx and the stomach, or intestine in
vertebrates that lack a stomach.
• The esophagus primarily transports food but
performs additional functions in some species.
– Crushing eggs in snakes, seed eating birds have a
specialized crop, a sac that develops caudally.
• It is lined by stratifies squamous epithelium that
is slightly keratinized in species that swallow
coarse foods (herbivores).
– The lining often bears papillae or longitudinal folds.
The Stomach
• Tunicates, amphioxus, ammocoetes larva,
hagfishes, and lamprey lack stomachs.
• Presumably a stomach evolved as vertebrates
began feeding on larger food particles that
were captured less frequently.
• The stomach, primarily, is a chamber for the
storage of ingested food.
• The stomach, usually, is a j-shaped sac.
• A pyloric sphincter at its posterior end normally is
contracted, so food is retained in the stomach
until it has been broken down.
• A cardiac sphincter sometimes is present at the
esophageal end to prevent food from being
regurgitated.
• Goblet cells are abundant throughout the lining,
and together with branched tubular glands in the
pyloric and cardiac regions, secrete a copious
amount of mucus to prevent autodigestion.
• Simple gastric glands secrete hydrochloric acid
and pepsinogen, which is altered to pepsin in the
stomach.
• Some verts have evolved specializations of the
stomach.
– The puffer and porcupine fishes have a chamber
that fills with water causes the animal to swell
greatly in size.
• In certain fishes, reptiles (including
crocodiles), and all birds a gizzard has formed.
– The gizzard has a thick muscular wall and a tough
lining, and usually contains small stones that have
been swallowed.
• It is important in animals that have lost their teeth or
do not chew foods that are hard to digest.
• A number of mammals have developed a
complex, chambered stomach that enables them
to process plant materials as food.
• Plants are abundant, but the energy within them
is low; therefore a large amount of material must
be consumed.
– This requires that some part of the gut have a large
capacity and that the passage of food is very slow to
allow colonies of microorganisms to ferment and
break down cellulose.
• Hippopotamuses, camels, giraffes, and ruminants all have
chambered stomachs.
– The general plan of a chambered stomach is; the
rumen, reticulum, omasum, and abomasum, in linear
sequence.
The Intestine and Cloaca
• Apart from the initial starch digestion that
occurs in the mouth of some verts, and initial
protein digestion in the stomach; the intestine
is the primary site for both digestion and
absorption.
• Secretions from both the liver and pancreas
are added to intestinal secretions and play an
important role in absorption.
• Digested organic materials, mineral ions,
water, and other small molecules are
absorbed from the intestinal lumen.
– Fatty acids and fat-soluble vitamins simply diffuse
through the plasma membrane of intestinal cells.
• The surface area of absorption varies greatly
among craniates according to the type of food
and level of activity and metabolism
• Undigested residues, together with bacteria
that have multiplied in the intestine, are
egested as feces.
• The intestine usually opens into a cloaca, a
chamber that also receives the excretory and
reproductive ducts.
• The cloaca opens to the body surface by a
cloacal aperture.
• A cloaca is absent in most teleosts and
mammals and in these vertebrates the
intestine terminates in an anus on the body
surface.
END