Chapter 12 Digestive System

PROCURING FOOD

Filter feeders: ancestral chordates (sea squirts, amphioxus), living larval agnathans (e.g. lampreys)

o This mode of feeding is primitiveo Can be employed only by aquatic organismso Passive filtration of organic matter from the incoming

respiratory stream, propel particles to the rear of the pharynx for swallowing

More active version of filter feeding in some fishes (spoonbills, cluepids, basking sharks)

o Plankton & small fish strained out of respiratory stream using long filamentous gill rakers hanging into pharyngeal chamber from gill arches

Whalebone/Baleen hangs into oral cacity Note that whales take water into their oral

cavity for the sole purpose of feeding; they DO NOT breathe with gills (water spills out of the mouth to be sent back to sea)

Active predation/active acquisition of particulate food in adults of early fishes -> led to paired external sense organs & their concentration on the headMore aggressive methods of obtaining food made possible by jaws & elaborate musculature of body walls (for locomotion & pursuit)

o Jaws were first invested with denticulated bony dermal armor

o In specialized predators, eventually furnished with sharp denticles (“teeth”)

Bite-tear-swallow technique, as in modern sharks

This required no tongue or other specializations of oral cavity

Less energy consuming procedure due to further adaptations of skull & hyoid arches

o Protrusible jaws, suction (as in goldfish)o Lampreys, due to parasitic nature, rasp host tissues with

spiny “tongue” (fleshy & cartilaginous rod armed w/ horny teeth) & suck the debris into pharynx

Carnivorous mammals: snap-bite-tear techniqueo Often involves piercing effect of saberlike tooth

Food taking depends on food findingo Use chemical, mechanical, thermal, capsulated touch,

visual, electroreceptors

THE DIGESTIVE TRACT: AN OVERVIEW

Digestive tract aka alimentary canalFrom mouth to vented cloaca or anus (directly exterior)Functions: digestion & absorption, elimination of undigested wastesPeristalsis: food propelled from pharynx to vent or anus due to action of smooth muscle tissue in walls of the tractMajor subdivisions:

o Oral cavity & pharynx (in fishes: oropharyngeal cavity)o Esophaguso Stomacho Intestine (divided into small & large in tetrapods)

Ducts from accessory organs also empty into the tracto Pancreas, liver, gall bladdero These organs arise as evaginations from the embryonic

digestive tractCeca are also commonly present

Digestive system = tract + accessory organso Occupies the pleuroperitoneal cavity in fishes,

amphibians, most reptileso Lungs occupy separate pleural cavities; digestive organs

beyond esophagus occupy abdominal/peritoneal cavity in mammals, birds, & few other reptiles

Coelom arises via delamination of early lateral-plate mesoderm into somatic & splanchnic mesoderm

Most of embryonic digestive tract EXCEPT the endodermal lining comes from splanchnic mesoderm (in cephalochordates & craniates)

Visceral peritoneum – outer covering of digestive tract, continuous with parietal peritoneum

o Early in embryonic life, parietal & visceral petionia continuous via dorsal & ventral mesenteries; coelom is divided into separate right & left cavities

o Dorsal mesentery remains intact (conducts blood vessels & nerves from roof of coelom to digestive organs)

o Ventral mesentery disappears EXCEPT at the level of the liver & urinary bladder

Embryonic digestive tract has 3 regions:o Midgut – contains the yolk (when present), where the

yolk sac is attached; little remains in adultso Hindgut – becomes remainder of intestine and cloacao Foregut – elongates to form part of oral cavity, pharynx,

esophagus, stomach, much of small intestineStomodeum – anterior portion of oral cavity, or in the case of fishes, oropharynx; arises as midventral invagination of ectoderm of the headOral plate – temporarily separates early embryonic foregut from exterior; ruptures to provide an anterior entrance to digestive tract

Proctodeum – counterpart of stomodeum in the hindut; provides the exit when the cloacal plate ruptures

o DeuterostomousDifferences in anatomy of digestive tracts caudal to pharynx correlated with nature & abundance of food

MOUTH & ORAL CAVITY

Mouth – entrance to digestive tracto opens into oropharyngeal cavity w/ teeth & walls

perforated by gill slits (in gnathostome fishes) Oropharyngeal cavity terminates at short

esophaguso Opens into oral/buccal cavity w/ teeth & tongue (in

tetrapods) Leads to the pharynx

Primary palate – roof of oropharyngeal cavity of fishes & amphibians

o Pierced anteriorly in lungfishes & amphibians by internal nares

Most Reptiles have an incomplete secondary palate, leaving in the roof of the oral cavity a palatal fissure that channelizes respiratory air between the choanae & pharynxCrocodilians & mammals: cleftless roof for oral cavity from mouth to pharynxAnurans: paired vocal sacs (reverberating chambers beneath floor of pharynx); tetrapods: oral glands or their ducts

o These open into the oral cavity :DOral vestibule – trench; in mammals; separates the gums (alveolar ridges) from the cheeks & lips

o Rodents: opening from vestibule leads into cheek pouch, used for transport

Extend from 1st cheek teeth to a position lateral to scapula

Lined with moderately low keratinized squamous epithelium, for protection vs. abrasion

Buccinators – retractor; a slip of this muscle inserts on the wall

Overlying skin is loose so that pouches can expand

o Seed & grain eating birds: median sublingual seed pouch Lies upon caudal portion of mylohyoid muscle

beneath oral cavity Retracted by homologue of genioglossus muscle

of mammals

Tongue

Elasmobranchs, bony fishes, perennibranchiate amphibians: crescentic or angular elevation of floor of oropharyngeal cavity shaped by basihyal & ceratohyal cartilages

o Primary tongue – lean hyoid elevation; no musculature, cannot be independently manipulated

May assist jaws in holding prey within oropharyngeal cavity

Forerunner of tetrapod tongueTerrestrial urodeles, anurans: primary tongue + extension that can be flipped out of mouth

o Primary tongue develops from hyoid arch mesenchymeo Extension develops from embryonic glandular field

anterior to hyoid arch Glandular field secretes sticky mucus that

entangles insects when tongue is suddenly thrust out of mouth

Insectivorous amphibians: tip of tongue usually terminates in an expansion that increases the probability that the prey will be caught & delivered into oral cavityTerrestrial urodeles: root of tongue anchored to basihyal & ceratohyal cartilages of pharyngeal floor :: anurans: unique! Floor of oral cavity immediately behind mandibular symphysis

o No tongue develops in toad family PipidaeReptiles & mammals:

o 3 distinct features Pair of lateral lingual swellings

From mesechyme from mandibular arch; not found outside of amniotes

Suppressed in birds Primary component from hyoid arch that

develops a glandular field Mesenchyme from 3rd pharyngeal arch spreads

forward over some of 2nd arch mesenchymeo Sensory epithelia thus formed from mesenchyme from

pharyngeal arches 1, 2, 3; innervated by cranial nerves 5, 7, & 9

o Hypobranchial musculature invades entire complex & receives somatic motor innervation from 12th cranial nerve

o Tongue of amniotes anchored to skeletal components of hyoid arch

o Turtles, crocs, alligators, some baleen whales: tongue also affixed to floor of oral cavity

o Garter snakes have no tongueBirds: tongue almost lacking in intrinsic muscles

o Only movement comes from muscular operation of hyoid skeleton to which tongue is anchored

Tongue of agnathans NOT homologous w/ any component of gnathostome tongue

o Rodlike lingual cartilage of unknown homology capped by horny spines

o Operated by protractor & retractor musclesTongue widely used to capture or gather food

o Flipping tongue (over mandibular symphysis) occurs when long fibers of genioglossus medialis muscle stiffen to form a complex of intrinsic rods & when genioglossus basalis muscle swells; returns to mouth via contraction of hypoglossal muscle

o Woodpeckers: barbed tongue; hummingbird tongue; immobilized tongue of baleen whales; parrot tongue armed w/ 2 flexible horny shields composed of keratinized epithelial cells

These epithelial cells grow forward from nail-like bed halfway back on tongue

Entoglossal bone – embedded within tongue of birds & lizards; anteriorly directed process of hyoidParaglossal bone – attached to entoglossus in many birdsTongues of most mammals protrusible though tied in floor of oral cavity by the frenulum linguaeStereognosis – perception of shape, weight, & texture of solid body

o Mucosa of amniote tongue contains receptors for thisEncapsulated nerve endings enable insectivores to search for food in dark places using their tongue; also allows seed-eating birds to manipulate seed being husked in the seed cupTongue manipulates fluids & solids; in most tetrapods, participates in swallowingAlso functions in cooling the blood; lizards clean their transparent eyelids/spectacles using their tongues; spiny papillae on tongue surface may be used by carnivores to rasp bones; may also be used by many mammals for grooming (kaya may hairballs hihi :D); speech

Oral Glands

Secretes watery or viscuous fluids Chief ingredient: mucus (varying viscosities & chemical composition)

o Moistens food to produce a bolus that can be manipulated by tongue

o Lubricates dry food for passage thru pharynx, down into esophagus

o Moisture also essential for taste buds to function (stimulant for taste must be in solution to evoke gustatory response)

o Other secretions include enzyme that digests starch (rare outside of mammals)

o Viscous secretions – keep tongue sticky; venom tranquilizes prey

Usually named according to locationo Labial glands – open into oral vestibule at base of the lipso Molar glandso Infraorbital – floor of orbito Palatal – includes venom glands of 4 families of

venomous snakes (venom exudes into groove/tube in the fang)

o Sublingual & submandibular – open via common papillae under tongue

In Heloderma, sublingual glands secrete toxino Intermaxillary/internasal – near premaxilla

Saliva – mixture of oral secretions; term usually reserved for oral secretions of mammals

o Parotid – largest tetrapod salivary gland Resembled histologically by poison gland of

reptileso Ptyalin (amylase) – one of the secretions of the parotid

o Specific mix of Mucus + serous secretions + ptyalin correlated with dietary habits

o Birds do not have a copious supply of salivaAquatic craniates commonly have mucus-producing goblet cells

o These are the only source of oral secretionso Mucus lubricates esophaguso Performs special function in male pop’n of few species of

catfishes Males carry fertilized eggs in brood pouches

(crypts) in the mucosa of the palate Goblet cells produce a copious nutritious

secretion that maintains a suitable env’t for the development of eggs; also nourishes the hatchlings

Brood pouches atrophy after hatchlings leave, in response to altered hormonal ratios

o Rare instance of multicellular oral gland: anticoagulant gland of lampreys

TeethBony teeth: jawed fishes, amphibians, reptiles, most mammals, earliest birds

o Achieved a peak in regional specialization in mammalsNo teeth: sturgeons, numerous teleosts including sea horses, few amphibians, all turtles, modern birds, whalebone whales, South American & scaly anteaters, Echidna

o Many toothless species still develop an embryonic set of teeth, but this doesn’t disrupt, or it disappears after disrupting

Descendant of denticles of dermal armor that covered the head & extended into the oropharyngeal cavity of early fishes

o Gnathostomes with no teeth have lost the genetic code necessary to induce or complete their development

Early fishes: bony dental plates of dermal origin overlay endoskeletal components of jaws

o Pointed, rounded, or jagged surface projections These prevented the escape of live food from

oropharyngeal cavity Used to crush shellfish To bite flesh To rasp vegetation

o Placoderms: pattern of dermal structures associated with jaws incompletely known

Range from unossified Meckelian cartilage associated with overlying denticles within skin to fully ossified dermal plates overlying or adjacent to Meckelian cartilage

Did NOT possess teeth Surface of dental plates hardened & shaped to

perform toothlike functions Morphology of biting surface of dental plates

correlated with putative function Flat – crushing Sharp – shearing Spikes – impaling Cusps – holding prey Loss of specialized biting structures in filter

feeders Single or paired upper dental plates

opposed the lower jaw on each sideo Acanthodians: each denticle attached directly to endoskeleton

of jaws, like modern teeth Jaws also often flanked by additional denticles Some toothless or had lower teeth

Dermal denticles and placoid scales show a gradual transition to teeth as they approach the cutting edges of jaws

o Proof that later vertebrate teeth are derived from bony dermal armor

Teeth are like placoid scales in that it is also composed of dentin (a variety of bone), surmounted by a crown of enamel or enameloidDevelopment

o Earliest indication of dev’t of socketed teeth: ingrowth into dermis of a longitudinal ridge of ectoderm, called the dental lamina

Extends more or less the length of the jawso Dermal papillae, each designating the site of a future tooth,

may be present beneath or above it; these papillae indent the lamina & organize blood vessels necessary for further development of a tooth primordium

o The cells at periphery of papilla become organized into a definite layer of odontoblasts that deposit dentin

o Odontoblasts slowly withdraw toward the center of the primordium (pulp cavity) as deposition proceeds

Evidence of withdrawal of odontoblasts: dentinal tubules that contain their protoplasmic processes

Odontoblasts remain alive throughout the life of the tooth

o Ectoderm of dental lamina organizes an enamel organ (ameloblasts)

This deposits enamel on the surface of dentin Thin layer of cementum (acellular bone) eventually

anchors tooth to bone of jaw via collagenous fiberso Living remnants of dermal papilla remain within whatever is left

of the pulp cavity (root canal) throughout the life of the tootho Details of tooth development and emergence, time of initiation

of different stages, ultimate fate of erupted teeth vary with the species

Armadillos & few other vertebrates: enamel organ present but functionlessMammals: enamel deposited by ameloblasts of ectodermal originFishes & other vertebrates: enameloids differ in physical characteristics, & the ultimate source of scleroblasts that elaborate them has been determined to be odontoblasts, which form a compact dentinGnathostomes: teeth vary in number, distribution within oral cavity, position with reference to summit of jaws, degree of permanence, shape

o Develop on jaws, palatal bones, & even pharyngeal skeleton in the oropharynx of living fishes

o Blue sucker has 35-40 teeth on the LAST gill arch 0__0Early tetrapods: teeth widely distributed on the palate

o Most amphibians & many reptiles have teeth on the vomer, palatine, & pterygoid bones; occasionally on the parasphenoid

Crocodilians, fossil toothed birds, mammals: confined to the jawso Least numerous ang teeth among mammals :Do Teeth, like dermal armor, have tended towards a more

restricted distribution with the passage of timeTypes of dentition

o Acrodont: as in many teleosts; teeth attached to outer surface or to summit of jawbone

o Pleurodont: as in anurans, urodeles, & many lizards; attached to inner side of jawbone

o Thecodont: occupy bone sockets or alveoli Sockets are deepest in mammals

o Polyphyodont: as in most gnathostomes through amniotes; succession of teeth, number of replacements during a lifetime indefinite but numerous

Elderly crocodile: replace front tooth 50x Crocodilians & other nonmammalian gnathostomes

replace teeth in waves that sweep along the jaws eliminating & replacing every other tooth

In one wave in tetrapods, even-numbered teeth are lost and odd-numbered ones in the next

Tooth germs for the next wave of eruptions subsequently form

Waves sweep in different directions in different species

Waves ensure a balanced distribution of teeth throughout life

Sharks: tooth germs form in dermis on the oropharyngeal cavity side of jaws; migrate onto cutting edge of jaw during growth; teeth that are being replaced move beyond the edge & fall away

o Diphyodont: 2 sets (deciduous/milk teeth & permanent teeth), definite sequence in which the teeth erupt

Only in mammals is there a definite number of teeth in a species

Sequence of eruption in humans (1-8 from front to rear): 6,1,2,4,5,3,7,8

Eruption of 8 is delayed in higher primates (wisdom tooth)

First set provides constantly changing infant jaw with small temporary teeth adequate for an infant’s diet until the jaws are more stabilized structurally & have elongated sufficiently to accommodate large teeth

o Monophyodont: 1st set only Platypus: milk teeth replaced by horny epidermal

teeth Toothless whales: 1st set forms within the jawbone;

these do not erupt or are shed if they do Freshwater manatee from the Amazon & Australian

Rock Wallaby: no sets, teeth replaced throughout life by the forward migration of new teeth formed at the rear of the jaws

Manatee: migration rate is 1 or 2 mm/month

Thin bony sockets separate roots of successive teeth, bony septa resorbed under pressure from migrating teeth

Grasses eaten by manatee contain abrasives that appear to be necessary for the teeth to move forward

Proboscidians: slow but constant succession of molar teeth that move forward from the rear

o Homodont: when all teeth are essentially similaro Heterodont: teeth vary morphologically from front to rear

Applies to all but a very few mammals (cetaceans, sirenians, & some marine carnivores exhibit homodont dentition)

Incisors, canines, premolars, molars (last 2 are cheek teeth)

Arose in the late synapsidsMorphologic variants in fishes

o Most sharks: fish eaters,; numerous rows of jaw teeth that are flat, sharp, or notched triangles used to cut; single or multipointed tusks that curve toward pharynx to hold struggling prey until it can be swallowed

Each shark tooth has a broad basal plate of bone embedded in dermis

Minority of sharks eat shellfish: teeth at entrance have curved caudally directed spines; rest are batteries of rounded denticles to crush shells

Tiny stomodeal denticles line the pharynx in some sharks (transitional shapes between denticles & teeth pag near the jaws)

o Holocephalans & modern lungfishes: similar to early jawed fishes; few large plates of enameloid/enamel-covered dentin that bear rows of various-sized rounded mound-like denticles which become sharp spines at the entrance to the oropharyngeal cavity

Chimaera: one large anterior + one small posterior dental plate on each side of upper jaw to cover the entire upper jaw; single large plate on each side below

Modern lungfish: plates restricted to palate & medial aspects of jaw

o Actinopterygii, amphibians, most reptiles: simple pointed cones attached to one or more membrane bones

Small teeth may be interspersed among larger ones Those in front sometimes larger & curved slightly to

rear Specialized shapes sometimes appear on one jaw or

the other Gars: few fanglike teeth shaped at their

ends like arrows

Venomous snakes: fangs borne on maxillae; curved, bladelike, or tubular for injecting venom

Morphologic variants in mammalso Incisors: located on either side of mandibular symphysis

One horizontal cutting edge Single root Best developed in herbivorous mammals (used to

hold, crop, or gnaw) Rodents & large front pair of lagomorphs: enamel on

anterior surface ONLY Lagomorphs have the 2nd pair of incisors

BEHIND the 1st

Gnawing wears away dentin faster; this keeps cutting edges of incisors sharp

Incisors grow throughout life Bovines: lacking on the upper jaw Vampire bats: lacking on the lower jaw Sloths: no incisors Elephant & mastodon tusks are modified incisors that

grow throughout life Walrus tusks NOT incisors but canines

o Canines: lie next to incisors Scarcely differ in appearance in generalized mammals Carnivores: spearlike, used to pierce flesh Absent in lagomorphs, so there is a diastema

between incisors & first cheek tooth Rodents: premolars are also missing so the diastema

is longer Attained greatest length on the upper jaw of saber-

toothed tigers 20 cm below lower jaw with the mouth

closed Lower canines reduced

o Premolars

Most mammals EXCEPT ungulates: 2 prominent cusps (bicuspid)

1 or 2 roots; number of roots may differ on upper & lower jaw among different individuals of same population

o Molars Tricuspid Usually with 3 roots; occasionally 4 or 5 Not replaced by a 2nd set; late arrivals of the 1st

o Crown: part of the tooth above the gum line Covered with enamel Crown of cheek teeth of carnivores & herbivorous

ungulates show extreme morphological differences Carnivores

o for tearing flesh & crushing boneo laterally compressed, 2 or 3 cusps

interconnected by sharp ridges of enamel, long roots (secodont)

o sharp enamel ridges of crowns produce shearing effect because the cusps of upper jaw teeth fit between the cusps of lower jaw teeth;

o carnassial teeth (last upper premolar + first lower molar) larger and longer in order to deal with tough shearing problems

Ungulates & some herbivoreso for macerating vegetationo cheek teeth wider & longer,

providing broad surfaces for grinding

o crowns are tall, allowing for plenty of wear

o crowns also consist of crescentic columns of dentin embedded in

additional dentin devoid of enamel overlay; each column surrounded by enamel

o Selenodonto Bovines

selenodont cheek teeth no teeth anterior to them in the upper jaw; cheek

teeth employed in chewing cudo Proboscidians

Adaptation for grinding exaggerated Lophodont teeth: enamel & dentin intricately

interfolded & enamel disposed on ridges on enormous plateaus of naked dentin

Reach a foot or more in length and a third of a foot in width in the largest elephants

o Remaining mammals Cheek teeth exhibit a variety of molariform styles Omnivores & some herbivores: low rounded cusps

instead of sharp edges and pointed cusps (bunodont teeth)

Humans, rhinos, some hogs, primitive ruminants, rodents

o Rodents Largest mammalian order, largest variety of diets,

largest variety of teeth Squirrels: low crowned with long roots Wood rats: high crowned with short roots

o Crabeater seal Among the most unusual mammalian teeth Employed to strain small crustaceans & other

plankton from mouthfuls of seawater as it spills back into the sea

o Early prototherians Triconodont: crown has 3 conelike prominences

arranged in a straight lineo Early therians

Trituberculate: crown’s 3 conelike prominences arranged in a triangle

Forerunners of today’s tricuspids Enamel crests connecting the cones is

thought to account for selenodont & lophodont teeth

1st therians: total of 44 teeth (3-1-4-3) See book for other formulas of a few

selected mammals :p (p. 279)Teeth, along with the tongue & hyoid, form a functional triad that procures, manipulates, & (in mammals) masticates foodstuffs at the entrance to the digestive tract, then starts a bolus of food

Epidermal TeethKeratinized (horny) teeth in living agnathans’ buccal cavity and tongue, for raspingAnuran tadpoles: temporary lips perched on poorly developed jaws, for rasping algae and other vegetation; shed and replaced by bony ones at metamorphosisTurtles, crocodilians, Sphenodon, birds, monotremes: temporary horny egg tooth used to crack egg shellPlatypus: horny teeth replace bony teeth and remain throughout lifeHorny beaks of turtles and modern birds have serrations that perform some of the functions of teeth

PHARYNXPart of the digestive tract that had pharyngeal pouches in the embryoOpens into the esophagusFishes: functional part of the respiratory systemMost constant features:

o Glottis – slit opening into larynxo Openings of paired auditory tubes – leads to the middle ear

cavityo Opening into the esophagus

In mammals, additional features:o Nasal pharynx above soft palate

nasal passageways empty into nasal pharynx via choanae

auditory tubes derived from 1st pair of pharyngeal pouches open into its lateral walls

pharyngeal tonsils (adenoids) develop in the mucosa o Oral pharynx between oral cavity and glottis

Isthmus of the fauces: narrow passageway marking transition from oral cavity to oral pharynx

Lateral walls of isthmus exhibit 2 pillars of the fauces, which are muscular folds that arch upward from the side of the tongue to the soft palate (glossopalatine arch) and from the pharyngeal wall to the soft palate (pharyngopalatine arch)

Humans & some other primates: uvula hangs from caudal border of soft palate into oral pharynx

Palatine tonsil in the hollow between the pillars Develop in the walls of the 2nd pharyngeal

pouch Remnant of pouch remains as pocket-like

crypt with tonsil at its wallo Laryngeal pharynx dorsal to the larynx in mammals in which the

opening to the esophagus is caudal to the glottiso Lingual tonsils develop on the tongue near its attachment to

the hyoid boneo Tonsils as lymphoid masses that serve as the body’s first line of

defense against infective agentso Epiglottis: fibrocartilaginous flap that lies in the floor of the

pharynx ventral to pharyngeal chiasma; attached to hyoid bone Regulation of air & food traffic

o In many mammals, swallowing draws the larynx forward (upward in humans) against the epiglottis, closing the glottis

o In other mammals, the epiglottis and part of the larynx can be drawn into the nasopharynx to provide an uninterrupted air pathway to the lungs; foodstuffs detour around the larynx & enter the esophagus

o Other tetrapods: fleshy valves at appropriate locations

E.g. valves that open/close the entrance to external nares in aquatic tetrapods

Some teleosts:o Suprabranchial organs (muscular tubes) evaginate from roof of

pharynx, terminate as blind sacs Elongated gill rakers from last 2 gill arches form

funnel-shaped baskets that extend into the entrance of suprabranchial organs

Each tube surrounded by cartilaginous capsules that furnishes attachment for the striated muscle of the tube

Epithelium at blind ends have many goblet cells Sacs contain quantities of plankton sometimes

compressed into a bolus Therefore, possible fxn: trap plankton from incoming

respiratory water stream & concentrate it into mucus-rich masses that are swallowed

Air-gulping teleost: cavity filled with air & highly vascularized epithelial lining serves as accessory respiratory membrane

MORPHOLOGY OF THE GUT WALL4 histological layers (outward): mucosa, submucosa, muscularis externa, serosa

o Differences lie in the thickness and the nature of glandsMucosa

o Consists chiefly of glandular epithelial lining of endodermal origin + layer of not very dense (areolar) connective tissue supporting base of cryptlike epithelial glands, lymph nodes, & blood and lymph capillaries + thin coat of smooth muscle fibers (muscularis mucosae, may be absent in some regions)

o Mucous glands provides a lubricant that facilitates passage of contents during peristalsis

Submucosa

o Thicker layer of connective tissue supporting base of compound alveolar glands & rich plexus of arterioles, venules, & lymphatics that service capillary beds of the mucosa

Muscularis externao Smooth muscle tissue arranged into 2 layers

Inner circular layer – smooth muscles, constricts lumen on neural demand

Outer longitudinal layer – contracts short segments of gut

o Combined action of smooth muscles produces the macerating, peristaltic, & (in mammalian colon) segmenting actions of the gut

o Neural stimuli for contraction supplied from autonomic (visceral) nerve plexuses between the longitudinal & circular muscle layers and between circular layer & submucosa

Serosao Consists of loose connective tissue (adventitia) + covering of

visceral peritoneumo Exudes small amounts of serous fluid that lubricates surface of

viscera, reducing frictiono Peritonitis: inflammation of serosa, leads to exudation of

excessive quantities of fluido Esophagus & caudalmost portion of intestine: covered by

serosa on one surface because they generally lie against the body wall

Many larval craniates, as in protochordates: Entire digestive tract ciliatedMany Teleosts: cilia in the stomachSome adult amphibians: cilia in the oral cavity, pharynx, esophagus, & stomachCilia are present for a time in the stomach of human fetusPeristalsis chiefly responsible for moving foodstuffs along alimentary canal