Organ system within humans and other animals
The
gastrointestinal tract
(
GI tract
,
digestive tract
,
alimentary canal
) is the tract or passageway of the
digestive system
that leads from the
mouth
to the
anus
. The GI tract contains all the major
organs
of the digestive system, in humans and other animals, including the
esophagus
,
stomach
, and
intestines
. Food taken in through the mouth is
digested
to extract
nutrients
and absorb
energy
, and the waste expelled at the anus as
faeces
.
Gastrointestinal
is an adjective meaning of or pertaining to the stomach and intestines.
Most animals
have a "through-gut" or complete digestive tract. Exceptions are more primitive ones:
sponges
have small pores (
ostia
) throughout their body for digestion and a larger dorsal pore (
osculum
) for excretion,
comb jellies
have both a ventral mouth and dorsal anal pores, while
cnidarians
and
acoels
have a single pore for both digestion and excretion.
[1]
[2]
The human gastrointestinal tract consists of the
esophagus
, stomach, and intestines, and is divided into the upper and lower gastrointestinal tracts.
[3]
The GI tract includes all structures between the
mouth
and the
anus
,
[4]
forming a continuous passageway that includes the main organs of digestion, namely, the
stomach
,
small intestine
, and
large intestine
. The complete
human digestive system
is made up of the gastrointestinal tract plus the accessory organs of digestion (the
tongue
,
salivary glands
,
pancreas
,
liver
and
gallbladder
).
[5]
The tract may also be divided into
foregut
,
midgut
, and
hindgut
, reflecting the
embryological
origin of each segment. The whole human GI tract is about nine meters (30 feet) long at
autopsy
. It is considerably shorter in the living body because the intestines, which are tubes of
smooth muscle tissue
, maintain constant
muscle tone
in a halfway-tense state but can relax in spots to allow for local distention and
peristalsis
.
[6]
[7]
The gastrointestinal tract contains the
gut microbiota
, with some 1,000 different
strains
of
bacteria
having diverse roles in the maintenance of
immune health
and
metabolism
, and many other
microorganisms
.
[8]
[9]
[10]
Cells of the GI tract release
hormones
to help regulate the digestive process. These
digestive hormones
, including
gastrin
,
secretin
,
cholecystokinin
, and
ghrelin
, are mediated through either
intracrine
or
autocrine
mechanisms, indicating that the cells releasing these hormones are conserved structures throughout
evolution
.
[11]
Human gastrointestinal tract
[
edit
]
Structure
[
edit
]
Illustration of
digestive system
The structure and function can be described both as
gross anatomy
and as
microscopic anatomy
or
histology
. The tract itself is divided into upper and lower tracts, and the intestines
small
and
large
parts.
[12]
Upper gastrointestinal tract
[
edit
]
The upper gastrointestinal tract consists of the
mouth
,
pharynx
,
esophagus
,
stomach
, and
duodenum
.
[13]
The exact demarcation between the upper and lower tracts is the
suspensory muscle of the duodenum
. This differentiates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe
gastrointestinal bleeding
as being of either "upper" or "lower" origin. Upon
dissection
, the duodenum may appear to be a unified organ, but it is divided into four segments based on function, location, and internal anatomy. The four segments of the duodenum are as follows (starting at the stomach, and moving toward the jejunum):
bulb
, descending, horizontal, and ascending. The suspensory muscle attaches the superior border of the ascending duodenum to the
diaphragm
.
The suspensory muscle is an important anatomical landmark that shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.
[14]
This is a thin muscle which is derived from the
embryonic
mesoderm
.
Lower gastrointestinal tract
[
edit
]
The lower gastrointestinal tract includes most of the
small intestine
and all of the
large intestine
.
[15]
In
human anatomy
, the
intestine
(
bowel
, or
gut
. Greek:
entera
) is the segment of the gastrointestinal tract extending from the pyloric sphincter of the
stomach
to the
anus
and as in other mammals, consists of two segments: the
small intestine
and the
large intestine
. In humans, the small intestine is further subdivided into the
duodenum
,
jejunum
, and
ileum
while the large intestine is subdivided into the
cecum
, ascending, transverse, descending, and sigmoid
colon
,
rectum
, and
anal canal
.
[16]
[17]
Small intestine
[
edit
]
The
small intestine
begins at the
duodenum
and is a tubular structure, usually between 6 and 7 m long.
[18]
Its
mucosal
area in an adult human is about 30 m
2
(320 sq ft).
[19]
The combination of the
circular folds
, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a total area of about 250 m
2
(2,700 sq ft) for the entire small intestine.
[20]
Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:
- Duodenum
: A short structure (about 20?25 cm long
[18]
) that receives
chyme
from the stomach, together with
pancreatic juice
containing
digestive enzymes
and
bile
from the
gall bladder
. The digestive enzymes break down proteins, and bile
emulsifies
fats into
micelles
. The
duodenum
contains
Brunner's glands
which produce a mucus-rich alkaline secretion containing
bicarbonate
. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in the chyme.
- Jejunum
: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8.2 ft) long and contains the
circular folds
also known as plicae circulares and
villi
that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
- Ileum
: The final section of the small intestine. It is about 3 m long, and contains
villi
similar to the jejunum. It absorbs mainly
vitamin B12
and
bile acids
, as well as any other remaining nutrients.
Large intestine
[
edit
]
The
large intestine
, also called the colon, forms an arch starting at the
cecum
and ending at the
rectum
and
anal canal
. It also includes the
appendix
, which is attached to the
cecum
. Its length is about 1.5 m, and the area of the mucosa in an adult human is about 2 m
2
(22 sq ft).
[19]
Its main function is to absorb water and salts. The colon is further divided into:
- Cecum
(first portion of the colon) and
appendix
- Ascending colon
(ascending in the back wall of the abdomen)
- Right colic flexure
(flexed portion of the ascending and transverse colon apparent to the
liver
)
- Transverse colon
(passing below the diaphragm)
- Left colic flexure
(flexed portion of the transverse and descending colon apparent to the
spleen
)
- Descending colon
(descending down the left side of the abdomen)
- Sigmoid colon
(a loop of the colon closest to the rectum)
- Rectum
- Anal canal
Development
[
edit
]
The gut is an
endoderm
-derived structure. At approximately the sixteenth day of human development, the
embryo
begins to fold
ventrally
(with the embryo's ventral surface becoming
concave
) in two directions: the sides of the embryo fold in on each other and the head and tail fold toward one another. The result is that a piece of the
yolk sac
, an
endoderm
-lined structure in contact with the
ventral
aspect of the embryo, begins to be pinched off to become the primitive gut. The yolk sac remains connected to the gut tube via the
vitelline duct
. Usually, this structure regresses during development; in cases where it does not, it is known as
Meckel's diverticulum
.
During
fetal
life, the primitive gut is gradually patterned into three segments:
foregut
,
midgut
, and
hindgut
. Although these terms are often used in reference to segments of the primitive gut, they are also used regularly to describe regions of the definitive gut as well.
Each segment of the gut is further specified and gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including the
stomach
and
colon
, develop as swellings or dilatations in the cells of the primitive gut. In contrast, gut-related derivatives ? that is, those structures that derive from the primitive gut but are not part of the gut proper, in general, develop as out-pouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.
[21]
Part
|
Part in adult
|
Gives rise to
|
Arterial supply
|
Foregut
|
esophagus to first 2 sections of the duodenum
|
Esophagus, stomach, duodenum (1st and 2nd parts), liver, gallbladder, pancreas, superior portion of pancreas
(Though the spleen is supplied by the
celiac trunk
, it is derived from dorsal mesentery and therefore not a foregut derivative)
|
celiac trunk
|
Midgut
|
lower duodenum, to the first two-thirds of the transverse colon
|
lower
duodenum
,
jejunum
,
ileum
,
cecum
,
appendix
,
ascending colon
, and first two-thirds of the
transverse colon
|
branches of the
superior mesenteric artery
|
Hindgut
|
last third of the transverse colon, to the upper part of the anal canal
|
last third of the
transverse colon
,
descending colon
,
rectum
, and upper part of the
anal canal
|
branches of the
inferior mesenteric artery
|
Histology
[
edit
]
General structure of the gut wall
The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.
[22]
The GI tract can be divided into four concentric layers in the following order:
Mucosa
[
edit
]
The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds the
lumen
, or open space within the tube. This layer comes in direct contact with digested food (
chyme
). The mucosa is made up of:
- Epithelium
? innermost layer. Responsible for most digestive, absorptive and secretory processes.
- Lamina propria
? a layer of connective tissue. Unusually cellular compared to most connective tissue
- Muscularis mucosae
? a thin layer of
smooth muscle
that aids the passing of material and enhances the interaction between the epithelial layer and the contents of the lumen by agitation and
peristalsis
The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium.
Submucosa
[
edit
]
The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and
muscularis externa
. It contains the
submucosal plexus
, an
enteric nervous plexus
, situated on the inner surface of the
muscularis externa
.
Muscular layer
[
edit
]
The
muscular layer
consists of an inner circular layer and a
longitudinal
outer layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.
[23]
Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food. The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer, and the outer longitudinal layer.
Between the circular and longitudinal muscle layers is the
myenteric plexus
. This controls peristalsis. Activity is initiated by the pacemaker cells, (myenteric
interstitial cells of Cajal
). The gut has intrinsic peristaltic activity (
basal electrical rhythm
) due to its self-contained enteric nervous system. The rate can be modulated by the rest of the
autonomic nervous system
.
[23]
The coordinated contractions of these layers is called
peristalsis
and propels the food through the tract. Food in the GI tract is called a bolus (ball of food) from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to as
chyme
. In the large intestine, the remaining semi-solid substance is referred to as
faeces
.
[23]
Adventitia and serosa
[
edit
]
The outermost layer of the gastrointestinal tract consists of several layers of
connective tissue
.
Intraperitoneal
parts of the GI tract are covered with
serosa
. These include most of the
stomach
, first part of the
duodenum
, all of the
small intestine
,
caecum
and
appendix
,
transverse colon
,
sigmoid colon
and
rectum
. In these sections of the gut, there is a clear boundary between the gut and the surrounding tissue. These parts of the tract have a
mesentery
.
Retroperitoneal
parts are covered with
adventitia
. They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the
transpyloric plane
. These include the
esophagus
,
pylorus
of the stomach, distal
duodenum
,
ascending colon
,
descending colon
and
anal canal
. In addition, the
oral cavity
has adventitia.
Gene and protein expression
[
edit
]
Approximately 20,000 protein coding genes are expressed in human cells and 75% of these genes are expressed in at least one of the different parts of the digestive organ system.
[24]
[25]
Over 600 of these genes are more specifically expressed in one or more parts of the GI tract and the corresponding proteins have functions related to digestion of food and uptake of nutrients. Examples of specific proteins with such functions are
pepsinogen PGC
and the
lipase LIPF
, expressed in
chief cells
, and gastric
ATPase ATP4A
and
gastric intrinsic factor GIF
, expressed in
parietal cells
of the stomach mucosa. Specific proteins expressed in the stomach and duodenum involved in defence include
mucin
proteins, such as
mucin 6
and
intelectin-1
.
[26]
Transit time
[
edit
]
The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender.
[27]
[28]
Several techniques have been used to measure transit time, including radiography following a
barium
-labeled meal, breath
hydrogen
analysis,
scintigraphic
analysis following a
radiolabeled
meal,
[29]
and simple ingestion and spotting of
corn kernels
.
[30]
It takes 2.5 to 3 hours for 50% of the contents to leave the stomach.
[
medical citation needed
]
The rate of digestion is also dependent of the material being digested, as food composition from the same meal may leave the stomach at different rates.
[31]
Total emptying of the stomach takes around 4?5 hours, and transit through the colon takes 30 to 50 hours.
[29]
[32]
[33]
Immune function
[
edit
]
The gastrointestinal tract forms an important part of the
immune system
.
[34]
Immune barrier
[
edit
]
The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court.
[19]
With such a large exposure (more than three times larger than the
exposed surface of the skin
), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems.
[35]
Fundamental components of this protection are provided by the
intestinal mucosal barrier
, which is composed of physical, biochemical, and immune elements elaborated by the intestinal mucosa.
[36]
Microorganisms also are kept at bay by an extensive immune system comprising the
gut-associated lymphoid tissue
(GALT)
There are additional factors contributing to protection from pathogen invasion. For example, low
pH
(ranging from 1 to 4) of the stomach is fatal for many
microorganisms
that enter it.
[37]
Similarly,
mucus
(containing
IgA
antibodies
) neutralizes many pathogenic microorganisms.
[38]
Other factors in the GI tract contribution to immune function include
enzymes
secreted in the
saliva
and
bile
.
Immune system homeostasis
[
edit
]
Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example,
Clostridia
, one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system.
[39]
It has been demonstrated that the intake of a high fiber diet could be responsible for the induction of
T-regulatory cells
(Tregs). This is due to the production of
short-chain fatty acids
during the fermentation of plant-derived nutrients such as
butyrate
and
propionate
. Basically, the butyrate induces the differentiation of Treg cells by enhancing
histone H3
acetylation
in the promoter and conserved non-coding sequence regions of the
FOXP3
locus, thus regulating the
T cells
, resulting in the reduction of the inflammatory response and allergies.
Intestinal microbiota
[
edit
]
The large intestine contains multiple types of
bacteria
that can break down molecules the human body cannot process alone,
[40]
demonstrating a
symbiotic
relationship. These bacteria are responsible for gas production at
host?pathogen interface
, which is released as
flatulence
. However, the primary function of the large intestine is water absorption from digested material (regulated by the
hypothalamus
) and the reabsorption of
sodium
and nutrients.
[41]
Beneficial
intestinal bacteria
compete with potentially harmful
bacteria
for space and "food", as the intestinal tract has limited resources. A ratio of 80?85% beneficial to 15?20% potentially harmful bacteria is proposed for maintaining
homeostasis
.
[
citation needed
]
An imbalanced ratio results in
dysbiosis
.
Detoxification and drug metabolism
[
edit
]
Enzymes
such as
CYP3A4
, along with the
antiporter
activities, are also instrumental in the intestine's role of
drug metabolism
in the detoxification of
antigens
and
xenobiotics
.
[42]
Other animals
[
edit
]
In most
vertebrates
, including
fishes
,
amphibians
,
birds
,
reptiles
, and
egg-laying mammals
, the gastrointestinal tract ends in a
cloaca
and not an
anus
. In the cloaca, the
urinary system
is fused with the genito-anal pore.
Therians
(all mammals that do not lay eggs, including humans) possess separate anal and uro-genital openings. The females of the subgroup
placentalia
have even separate urinary and genital openings.
During
early development
the asymmetric position of the bowels and inner organs is initiated (see also
axial twist theory
).
Ruminants
show many specializations for digesting and
fermenting
tough plant material, consisting of
additional stomach compartments
.
Many birds and other animals have a specialised stomach in the digestive tract called a
gizzard
used for grinding up food.
Another feature found in a range of animals is the
crop
. In birds this is found as a pouch alongside the esophagus.
In 2020, the oldest known fossil digestive tract, of an extinct wormlike organism in the
Cloudinidae
was discovered; it lived during the late
Ediacaran
period
about 550 million years ago.
[43]
[44]
A through-gut (one with both mouth and anus) is thought to have evolved within the
nephrozoan
clade of
Bilateria
, after their ancestral ventral orifice (single, as in
cnidarians
and
acoels
; re-evolved in nephrozoans like
flatworms
) stretched antero-posteriorly, before the middle part of the stretch would get narrower and closed fully, leaving an anterior orifice (mouth) and a posterior orifice (anus plus
genital opening
). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinct
proarticulates
. This and the
amphistomic
development (when both mouth and anus develop from the gut stretch in the embryo) present in some nephrozoans (e.g.
roundworms
) are considered to support this hypothesis.
[45]
[46]
Clinical significance
[
edit
]
This section discusses related diseases, medical associations with the gastrointestinal tract, and use in surgery.
Diseases
[
edit
]
There are many diseases and conditions that can affect the gastrointestinal system, including
infections
,
inflammation
and
cancer
.
Various
pathogens
, such as
bacteria
that cause
foodborne illnesses
, can induce
gastroenteritis
which results from
inflammation
of the stomach and small intestine.
Antibiotics
to treat such bacterial infections can decrease the
microbiome
diversity of the gastrointestinal tract, and further enable inflammatory mediators.
[47]
Gastroenteritis is the most common disease of the GI tract.
Diverticular disease
is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well.
Diverticulosis
occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as
diverticulitis
.
Inflammatory bowel disease
is an inflammatory condition affecting the bowel walls, and includes the subtypes
Crohn's disease
and
ulcerative colitis
. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an
autoimmune disease
. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such.
Functional gastrointestinal disorders
the most common of which is
irritable bowel syndrome
. Functional constipation and
chronic functional abdominal pain
are other functional disorders of the intestine that have physiological causes but do not have identifiable structural, chemical, or infectious pathologies.
Symptoms
[
edit
]
Several symptoms can indicate problems with the gastrointestinal tract, including:
Treatment
[
edit
]
Gastrointestinal surgery
can often be performed in the outpatient setting. In the United States in 2012, operations on the digestive system accounted for 3 of the 25 most common ambulatory surgery procedures and constituted 9.1 percent of all outpatient ambulatory surgeries.
[48]
Imaging
[
edit
]
Various methods of imaging the gastrointestinal tract include the
upper
and
lower gastrointestinal series
:
Other related diseases
[
edit
]
- Cholera
- Enteric duplication cyst
- Giardiasis
- Pancreatitis
- Peptic ulcer disease
- Yellow fever
- Helicobacter pylori
is a
gram-negative
spiral bacterium. Over half the world's population is infected with it, mainly during childhood; it is not certain how the disease is transmitted. It colonizes the gastrointestinal system, predominantly the stomach. The bacterium has specific survival conditions that are specific to the human gastric
microenvironment
: it is both
capnophilic
and
microaerophilic
.
Helicobacter
also exhibits a
tropism
for gastric epithelial lining and the gastric mucosal layer about it. Gastric colonization of this bacterium triggers a robust immune response leading to moderate to severe
inflammation
, known as
gastritis
. Signs and symptoms of infection are gastritis, burning abdominal pain, weight loss, loss of appetite, bloating, burping, nausea, bloody vomit, and black tarry stools. Infection can be detected in a number of ways: GI X-rays, endoscopy, blood tests for anti-
Helicobacter
antibodies, a stool test, and a urease breath test (which is a by-product of the bacteria). If caught soon enough, it can be treated with three doses of different proton pump inhibitors as well as two antibiotics, taking about a week to cure. If not caught soon enough, surgery may be required.
[49]
[50]
[51]
[52]
- Intestinal pseudo-obstruction
is a syndrome caused by a malformation of the digestive system, characterized by a severe impairment in the ability of the intestines to push and assimilate. Symptoms include daily abdominal and stomach pain, nausea, severe distension, vomiting, heartburn, dysphagia, diarrhea, constipation, dehydration and malnutrition. There is no cure for intestinal pseudo-obstruction. Different types of surgery and treatment managing life-threatening complications such as ileus and volvulus, intestinal stasis which lead to bacterial overgrowth, and resection of affected or dead parts of the gut may be needed. Many patients require parenteral nutrition.
- Ileus
is a blockage of the intestines.
- Coeliac disease
is a common form of
malabsorption
, affecting up to 1% of people of northern European descent. An autoimmune response is triggered in intestinal cells by digestion of gluten proteins. Ingestion of proteins found in wheat, barley and rye, causes villous atrophy in the small intestine. Lifelong dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment.
- Enteroviruses
are named by their transmission-route through the intestine (
enteric
meaning intestinal), but their symptoms are not mainly associated with the intestine.
- Endometriosis
can affect the intestines, with similar symptoms to IBS.
- Bowel twist
(or similarly, bowel strangulation) is a comparatively rare event (usually developing sometime after major bowel surgery). It is, however, hard to diagnose correctly, and if left uncorrected can lead to bowel
infarction
and death. (The singer
Maurice Gibb
is understood to have died from this.)
- Angiodysplasia
of the colon
- Constipation
- Diarrhea
- Hirschsprung's disease
(aganglionosis)
- Intussusception
- Polyp (medicine)
(see also
colorectal polyp
)
- Pseudomembranous colitis
- Toxic megacolon
usually a complication of ulcerative colitis
Uses of animal guts
[
edit
]
Intestines from animals other than humans are used in a number of ways. From each species of
livestock
that is a source of
milk
, a corresponding
rennet
is obtained from the intestines of milk-fed
calves
.
Pig
and
calf
intestines are eaten, and pig intestines are used as
sausage
casings. Calf intestines supply
calf-intestinal alkaline phosphatase
(CIP), and are used to make
goldbeater's skin
.
Other uses are:
- The use of animal gut
strings
by musicians can be traced back to the
third dynasty of Egypt
. In the recent past, strings were made out of
lamb
gut. With the advent of the modern era, musicians have tended to use strings made of
silk
, or synthetic materials such as
nylon
or
steel
. Some instrumentalists, however, still use gut strings in order to evoke the older tone quality. Although such strings were commonly referred to as "
catgut
" strings,
cats
were never used as a source for gut strings.
[53]
- Sheep gut was the original source for natural gut string used in
racquets
, such as for
tennis
. Today, synthetic strings are much more common, but the best gut strings are now made out of
cow
gut.
- Gut cord has also been used to produce strings for the snares that provide a
snare drum
's characteristic buzzing timbre. While the modern snare drum almost always uses metal wire rather than gut cord, the
North African
bendir
frame drum still uses gut for this purpose.
- "Natural"
sausage
hulls, or
casings
, are made of animal gut, especially hog, beef, and lamb.
- The wrapping of
kokoretsi
,
gardoubakia
, and
torcinello
is made of lamb (or goat) gut.
- Haggis
is traditionally boiled in, and served in, a sheep stomach.
- Chitterlings
, a kind of food, consist of thoroughly washed
pig
's gut.
- Animal gut was used to make the cord lines in
longcase clocks
and for
fusee
movements in
bracket clocks
, but may be replaced by metal wire.
- The oldest known
condoms
, from 1640 AD, were made from animal intestine.
[54]
See also
[
edit
]
References
[
edit
]
- ^
"Overview of Invertebrates"
.
www.ck12.org
. 6 October 2015
. Retrieved
25 June
2021
.
- ^
Ruppert EE, Fox RS, Barnes RD (2004). "Introduction to Bilateria".
Invertebrate Zoology
(7 ed.). Brooks / Cole. p. 197
[1]
.
ISBN
978-0-03-025982-1
.
- ^
"gastrointestinal tract"
at
Dorland's Medical Dictionary
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