Digestive system of vertebrates

1. DIGESTIVE
SYSTEM OF
VERTEBRATES
Comparative Anatomy
By-SanjuSah
St.Xavier’sCollege,Maitighar,Kathmandu
Department of Microbiology

2. Digestive System
■ Digestive system = Digestive tract/ALIMENTARY CANAL
+
Associated accessory organs
■ Digestive tract = Mouth, buccal cavity, pharynx,
oesophagus, stomach, small intestine, large
intestine and cloaca/anus.
■ Associated organs = Teeth, tongue, oral glands,
liver, pancreas, gall bladder etc.
Function: Ingestion, peristalsis, digestion, absorption,
assimilation, egestion

3. Embryonic Development of Digestive Tract
 One of the earliest system to form during embryonic
development.
 Embryonic archenteron formed of endoderm becomes
the lining of adult digestive tract and its derivatives.
 Splanchnic mesoderm adds connective tissue and
smooth (involuntary) muscle layers around the
archenteron/ primitive gut.

4. Digestive System (development)
■ The part of archenteron/primitive gut containing yolk or
connected by a narrow stalk with yolk sac is called midgut.
■ The part anterior to midgut is foregut and posterior to midgut
is hindgut.
■ Fore gut is differentiated into – oral cavity, pharynx,
oesophagus, stomach and most of the part of small intestine
of the adult body.
■ Hindgut forms large intestine and cloaca.

6. Digestive System (Development)
■ Vertebrates are deuterostomes.
■ A midventral ectodermal caudal invagination called
proctodaeum forms the external part of cloaca or
anus.
■ Similar midventral ectodermal anterior invagination
called stomodaeum forms the buccal cavity. It gives
rise to enamel of teeth, epithelial lining of oral cavity
and different oral glands like- poison, salivary, mucus
glands etc.

7. Digestive System (Histology)
Histology of wall of Digestive Tract:
■ Wall of alimentary canal of vertebrates bears 4 concentric layers:
serosa, muscularis, sub-mucosa and mucosa.
a. Serosa/visceral peritoneum: the outermost layer made of
squamous mesodermal cells.
b. Muscularis: Smooth muscle fibres arranged in outer longitudinal
and inner circular muscle layers. Autonomic nerve network
(myenteric plexus) is present between them.
c. Sub-mucosa: It is a connective tissue layer containing elastic
fibres, blood vessels, lymph vessels, nerves and glands.
d. Mucosa: It is the innermost layer, further differentiated into 3
layers: muscularis mucosa, lamina propria and epithelium.

10. ■ Outer muscularis mucosa with thin layers of longitudinal and
circular muscles.
■ Middle, lamina propria/corium having connective tissue, blood
capillaries, lacteals and nerves.
■ Innermost, mucosal epithelium made of columnar cells
resting on basement membrane; often ciliated or glandular

11. Digestive System of Vertebrates
1. Mouth- the anterior opening
■ Agnatha/cyclostomes: circular opening at the apex
of buccal funnel. It remains permanently open in the
absence of jaws or other closing mechanism.
■ Gnathostomes:
- Elasmobranchs and sturgeons have ventral mouth,
all others have terminal mouth.
- In fishes, amphibians and most of reptiles, mouth is
surrounded by unmodified or heavily cornified
immovable lips.
- In birds, mouth is wide and bounded by horny
beaks.
- In mammals, mouth is surrounded by fleshy and
movable lips.

12. Digestive System of Vertebrates
2. Buccal Cavity/ Oral Cavity:
■ In cyclostomes, buccal funnel is cup-like depression acting as a
sucker.
■ In fishes, oral cavity is shallow, loosely organized serving as
passageway for food and also for water helping in aquatic respiration.
It is quite independent of nasal cavities.
■ In amphibians and lower reptiles oral cavity is much compact,
muscular floor serves for swallowing food and also used in bucco-
pharyngeal respiration and pulmonary breathing in the absence of
diaphragm.
■ In Dipnoi, amphibians and most reptiles, nasal cavities open into
oral cavity by a pair of internal nares or choanae at anterior region.

14. Digestive System of Vertebrates
■ In birds nasal and oral cavities are in direct communication
as the palate (roof of buccal cavity) is cleft.
■ In mammals, oral cavity is highly specialized for efficient
sucking and chewing.
■ In higher reptiles and mammals, internal nares open far
posterior into pharynx.
■ In mammals, bony palate separating buccal cavity and nasal
cavities is continued posteriorly as a membranous soft palate
and often a fleshy pendulant process – the uvula, hangs
down.

15. Digestive System of Vertebrates
Accessory organs of Buccal cavity:
a. Tongue
■ Almost all vertebrates bear tongue.
■ Vertebrate tongue shows much diversity and is not homologous in all.
■ In cyclostomes: it is thick, fleshy extensible structure on the buccal
floor bearing horny epidermal teeth for rasping skin and muscles of
the prey.
■ In fishes: it is immobile, non-muscular sensory elevation on buccal
floor bearing receptors and also teeth in some bony fishes.
■ Most of amphibians- bear sticky tongue attached at the anterior end
and free at posterior end. It can be thrust out (protrusible) suddenly by
rapid injection of lymph into it for capturing prey.

16. Fish eating
the worm
Tongue of
fish
Cyclostome
Tongu
e

17. Digestive System of Vertebrates
■ Chelonian and crocodilian reptiles, some birds and
whales have immobile tongue.
■ Squamatan reptile (snake & lizards), some birds (eg.
Woodpecker) and some mammals (eg. Anteaters) bear
long, highly protrusible tongue often used for food
capture.
■ In most mammals, base of tongue is attached with the
floor of buccal cavity by fold of mucosal membrane
called frenulum.
■ In mammals, upper surface of tongue bears 4 types of
papillae providing roughness, some of which bear taste
buds. They are circumvalate papillae, fungiform

18. Echidna

19. Tongue of Human

20. Digestive System of Vertebrates
b. Teeth
■ 2 types: epidermal teeth and true teeth.
■ Cyclostomes bear epidermal teeth which are horny projections
of stratum corneum on the wall of buccal funnel and tongue.
■ True teeth occur in all vertebrates except agnathans, sturgeons,
some toads, sirens, turtles, modern birds etc.
■ In fishes and amphibians: dentition is acrodont, homodont and
polyphyodont.
■ In reptiles: it is pleurodont, homodont and polyphyodont.
■ In mammals, it is thecodont, heterodont and diphyodont
(monophyodont in moles and Indian squirrel).

21. Pleurodont (Reptiles)
Thecodont and heterodont
Acrodont (frog)

22. Digestive System of Vertebrates
c. Oral Glands:
■ Vertebrates bear variety of glands opening into oral cavity.
■ Oral glands are absent or few in aquatic forms, gradually increase in
terrestrial form.
■ Fishes and aquatic amphibians bear only simple mucous glands.
■ In poisonous snakes, upper labial glands produce poison.
■ Birds have sublingual glands and a gland in the angle of the mouth.
■ Mammals have many small mucous glands and true and enlarged
enzymatic salivary glands – parotid, sublingual, submaxillary and
infraorbital salivary glands secreting mucin and ptyalin.

24. Pharynx
■ Part of foregut between buccal cavity and
oesophagus.
■ From embryonic pharynx spiracles, gill clefts,
lungs, air bladder, tonsils, thyroid, parathyroid
and thymus are derived.
■ It is the common passage for digestive and
respiratory tracts.

25. Pharynx
■ In cyclostomes: In Petromyzon only respiratory pharynx is
present which is a blind pouch bearing gill slits while in Myxine it
is continued into oesophagus.
■ In fishes, it is extensive and perforated by gill slits for aquatic
respiration.
■ In tetrapods, it is short and common passage for digestive and
respiratory tract.
■ Tetrapods:
– pharynx is the part of the foregut preceding the esophagus
& includes:
■ Glottis – a slit leading into the larynx
■ openings of auditory (eustachian) tubes
■ Gullet - opening into esophagus

26. Oesophagus
■ Muscular, distensible tube between pharynx and stomach
that passes food by peristalsis.
■ Its length is related to the length of the neck.
■ In cyclostomes, it is a long slender tube opening directly
into intestine.
■ In anamniotes or neckless vertebrates (fishes and
amphibians)- very short.
■ In amniotes (reptiles, birds and mammals) – long.
■ It internally bears longitudinal folds; in elasmobranchs it is
lined by fleshy papillae and in marine turtles by horny
papillae.

27. Oesophagus
■ In graminivorous birds it
forms paired or unpaired food
storage sac called crop.
■ In pigeon, from crop of both
the sexes a curdy substance
called pigeons milk is
produced under the influence
of prolactin hormone from
pituitary gland.
■ In mammals, oesophagus is
long and enters from thoracic
cavity into abdominal cavity
piercing diaphragm at a point
called oesophageal hiatus.
Digestive System of Bird

28. Stomach
■ It is sac like part of digestive tract between oesophagus and
intestine.
- Temporary storage organ of food.
- Churning/maceration of food into fine pieces.
- chemical digestion of food.
■ True stomach bears gastric glands on epithelial lining.
■ Cyclostomes, chimaeras, lung fishes and some primitive
teleosts do not bear true stomach.
■ Elasmobranchs, most of teleosts and tetrapods bear well
developed stomach.
■ Stomach is sphinctered at both the ends; anterior- cardiac
sphincter and posterior- pyloric sphincter.

29. Stomach
■ Embryonic stomach is straight tube.
■ Lower vertebrates like gar, salamander etc bear straight
stomach throughout the life.
■ Most of the amphibians and squamatan reptiles bear
elongated and spindle shaped stomach.
■ Chelonian reptiles bear wide curved tubular stomach.
■ In crocodilian and most of the birds it is divisible into
anterior glandular thin walled proventriculus and
posterior thick walled highly muscular ventriculus or
gizzard.
■ Gizzard has inner tough protective lining and also bears
pebbles which macerate food.

31. Stomach
■ In mammals, there is greatest diversity in stomach.
■ May be a single sac.
■ May be divided into cardiac, fundic and pyloric region
with characteristic glands.
■ In ruminants (cud-chewing mammals), 4 distinct
compartments: rumen, reticulum, omasum and
abomasum.
■ Rumen, reticulum and omasum are modifications of
oesophagus, serve as reservoir.
■ Abomasum/rennet is true stomach with gastric glands.
■ In camel, omasum is absent.
■ In sanguivorous bats like Vampires, pyloric enlargement
serves to store blood.

32. Human Stomach
Ruminant Stomach

33. Stomach
■ Ruminant Mammals (Cud-chewing Ungulates)
– Possess ruminant stomach with 4 chambers.
■ When food is eaten it enters rumen and reticulum which
reduce the food to pulp.
– Microorganisms are present that aid in the breakdown
of complex carbohydrates in plant material.
■ The cud is then regurgitated for more chewing.
■ After chewing the cud, the remasticated material passes to
omasum and abomasum where physical and chemical
processing similar to normal mammalian stomach occurs.

35. Small Intestine
■ Principal site for digestion (completed) and absorption
(virtually almost all food is absorbed).
■ In cyclostomes, elasmobranchs and primitive bony
fishes - it is short, straight and wide tube opening into
cloaca. It bears typhlosole (a large internal fold) or
spiral valve increasing surface area for absorption.
■ Teleost and tetrapod intestine is devoid of spiral valve,
greatly elongated and coiled. Internal surface bears
small finger like villi.
■ Small intestine is differentiated into duodenum and
ileum.

36. Small intestine
■ Anteriormost widest part – duodenum bears Brunners
glands secreting alkaline mucus and crypts of
Lieberkuhn secreting intestinal juice containing
enzymes.
■ In mammals, behind the duodenum small intestine is
differentiated into jejunum and ileum.
■ Increased surface area in Tetrapods is by elongation and
coiling of intestine along with folding of internal surfaces
■ Intestine is longer in herbivores than in carnivores
because plant matter is more difficult to digest

37. Large Intestine
■ In cyclostomes, intestine is not
differentiated into small and large
intestine.
■ In most of fishes and amphibians,
it is wider than small intestine,
straight, short and leads into
cloaca (teleosts bear anus).
■ In reptiles, birds and mammals – it
is longer and divided into proximal
colon and distal rectum opening
into cloaca (except in mammals).
■ All mammals except monotremes
and almost all teleost fishes lack
cloaca, their rectum opens out
through anus.

39. Large Intestine
■ In fishes, ileocaecal valve is absent.
■ In tetrapods, ileo-caecal valve is present that prevents entry of
bacteria from colon to ileum.
■ In amniotes, a caecum is present; birds bear a pair of them.
■ Primates have small caecum with vestigeal vermiform appendix.
■ Herbivorous mammals bear long caecum and large functional
appendix with symbiotic bacteria helping in digestion of cellulosic
food.
■ Elasmobranchs bear a caecum and a rectal gland that secrets
Sodium chloride.

40. Digestive Tract of Anamniote
Vertebrates

41. Digestive Tract of

42. Digestive Glands:
Liver
■ The largest gland of the body of all vertebrates.
■ Arises as ventral outgrowth/diverticulum from the floor
of embryonic duodenum and divides into anterior and
posterior parts.
■ Anterior part proliferates and gives rise to liver proper
and bile ducts.
■ Posterior part gives rise to gall bladder, cystic duct and
common bile duct or ductus Choledochus.
■ Liver is a network of endodermal epithelial cells but
soon mesodermal derivatives incorporate with those to
form the adult organ.

43. Embryonic Development of Liver, gall-bladder and
pancreas

44. Liver
■ Vertebrate liver differs in shape, color and division into lobes.
■ In cyclostomes, it is small. Single lobed in lampreys and bilobed in
hagfishes.
■ In fishes, urodel amphibians and reptiles it is elongated, narrow and
cylindrical.
■ In elasmobranch fishes, it is bilobed.
■ In teleost fishes it is 2 or 3 lobed and may be green, yellow, orange,
brown or even red.
■ In amphibians, reptiles and birds it is 2 or 3 lobed, while its lobes
differ greatly in mammals. Human liver has 2 lobes while that of
rabbit has 5 lobes etc.

45. of Human

47. Liver
■ Liver cells secrete bile juice which is stored in gall bladder.
Bile juice neutralizes acidic chyme and emulsifies fats.
■ Liver is the vital organ for heat production, metabolic actions
like glycogenesis, glycogenolysis, gluconeogenesis,
lipogenesis, desaturation of fats, deamination, transamination
etc, synthesis of vitamins, phagocytosis of germs,
detoxification of drugs, alcoholic products & toxins etc.
■ A gall bladder is present in hagfishes (cyclostome) and all
higher vertebrates.
■ Gall bladder is absent in lampreys, many birds, aquatic
mammals (Cetacea) and some artioactyla and all
perissodactyla.
■ Gall bladder stores and concentrates bile juice.

48. Pancreas
■ The second largest gland.
■ Heterocrine/mixed
gland/compound gland
■ It is formed from one dorsal
and one or two ventral
diverticulum from
embryonic duodenum.
■ Distal portions of diverticula
divide and form compound
pancreas, one dorsal
pancreas and one ventral
pancreas.

49. Pancreas
 In fishes both dorsal and ventral pancreas persist
while in tetrapods two unite to form single
pancreas.
 In lampreys, some teleosts, lung fishes and lower
tetrapods it is probably exocrine only and
distributed diffusely in liver, mesentries and
intestinal wall.
 In elasmobranchs, most of teleosts and higher
tetrapods it is well-defined and compact.

51. Pancreas
■ Its exocrine portion consists of pancreatic acini
secreting digestive juice and endocrine portion consists
of Islets of Langerhans secreting hormones.
■ Pancreatic juice from acini is carried by the ducts which
unite to form major pancreatic duct (usually one in
higher vertebrates). Major duct joins with common bile
duct forming hepato-pancreatic duct that opens into
duodenum.
■ Islets of Langerhans bear endocrine cells like alfa cell,
beta cell, delta cell and PP cell or F cell) which
secrete hormones- glucagon, insulin, somatostatin and
pancreatic polypeptide hormone respectively. Glucagon
and insulin are involved in glucose metabolism.