Compa Ana Digestive System

Chapter 12 Digestive System

PROCURING FOOD 

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Filter feeders: ancestral chordates (sea squirts, amphioxus), living larval agnathans (e.g. lampreys) o This mode of feeding is primitive o Can be employed only by aquatic organisms o 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 head More 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 technique o Often involves piercing effect of saberlike tooth Food taking depends on food finding o Use chemical, mechanical, thermal, capsulated touch, visual, electroreceptors 

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THE DIGESTIVE TRACT: AN OVERVIEW     

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Digestive tract aka alimentary canal From mouth to vented cloaca or anus (directly exterior) Functions: digestion & absorption, elimination of undigested wastes Peristalsis: food propelled from pharynx to vent or anus due to action of smooth muscle tissue in walls of the tract Major subdivisions: o Oral cavity & pharynx (in fishes: oropharyngeal cavity) o Esophagus o Stomach o Intestine (divided into small & large in tetrapods) Ducts from accessory organs also empty into the tract o Pancreas, liver, gall bladder o These organs arise as evaginations from the embryonic digestive tract Ceca are also commonly present

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Digestive system = tract + accessory organs o Occupies the pleuroperitoneal cavity in fishes, amphibians, most reptiles o 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 adults o Hindgut – becomes remainder of intestine and cloaca o Foregut – elongates to form part of oral cavity, pharynx, esophagus, stomach, much of small intestine Stomodeum – anterior portion of oral cavity, or in the case of fishes, oropharynx; arises as midventral invagination of ectoderm of the head

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Oral 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 Deuterostomous Differences in anatomy of digestive tracts caudal to pharynx correlated with nature & abundance of food

MOUTH & ORAL CAVITY 

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Mouth – entrance to digestive tract o opens into oropharyngeal cavity w/ teeth & walls perforated by gill slits (in gnathostome fishes)  Oropharyngeal cavity terminates at short esophagus o 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 & pharynx Crocodilians & mammals: cleftless roof for oral cavity from mouth to pharynx Anurans: paired vocal sacs (reverberating chambers beneath floor of pharynx); tetrapods: oral glands or their ducts o These open into the oral cavity :D Oral 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

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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 & grain eating birds: median sublingual seed

Seed pouch  Lies upon caudal portion of mylohyoid muscle beneath oral cavity  Retracted by homologue of genioglossus muscle of mammals

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Tongue 

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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 tongue Terrestrial urodeles, anurans: primary tongue + extension that can be flipped out of mouth o Primary tongue develops from hyoid arch mesenchyme o 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

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Insectivorous amphibians: tip of tongue usually terminates in an expansion that increases the probability that the prey will be caught & delivered into oral cavity Terrestrial 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 Pipidae Reptiles & 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 mesenchyme o 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 12 th 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 tongue Birds: 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

Rodlike lingual cartilage of unknown homology capped by horny spines o Operated by protractor & retractor muscles Tongue 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 naillike bed halfway back on tongue Entoglossal bone – embedded within tongue of birds & lizards; anteriorly directed process of hyoid Paraglossal bone – attached to entoglossus in many birds Tongues of most mammals protrusible though tied in floor of oral cavity by the frenulum linguae Stereognosis – perception of shape, weight, & texture of solid body o Mucosa of amniote tongue contains receptors for this Encapsulated nerve endings enable insectivores to search for food in dark places using their tongue; also allows seedeating birds to manipulate seed being husked in the seed cup Tongue manipulates fluids & solids; in most tetrapods, participates in swallowing Also 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 o

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Oral Glands

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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 location o Labial glands – open into oral vestibule at base of the lips o Molar glands o Infraorbital – floor of orbit o 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 toxin o 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 reptiles o Ptyalin (amylase) – one of the secretions of the parotid

Specific mix of Mucus + serous secretions + ptyalin correlated with dietary habits o Birds do not have a copious supply of saliva Aquatic craniates commonly have mucus-producing goblet cells o These are the only source of oral secretions o Mucus lubricates esophagus o 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 o

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Teeth 

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Bony teeth: jawed fishes, amphibians, reptiles, most mammals, earliest birds o Achieved a peak in regional specialization in mammals No 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

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 side o 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 o

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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 enameloid Development 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 jaws o 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 fibers

Living remnants of dermal papilla remain within whatever is left of the pulp cavity (root canal) throughout the life of the tooth o 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 functionless Mammals: enamel deposited by ameloblasts of ectodermal origin Fishes & 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 dentin Gnathostomes: 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__0 Early 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 jaws o Least numerous ang teeth among mammals :D o Teeth, like dermal armor, have tended towards a more restricted distribution with the passage of time Types 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 o

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Sockets are deepest in mammals 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, evennumbered teeth are lost and oddnumbered 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 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 

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an infant’s diet until the jaws are more stabilized structurally & have elongated sufficiently to accommodate large teeth 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 Homodont: when all teeth are essentially similar 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 synapsids

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Morphologic 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/enamelcovered 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

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

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Bovines selenodont cheek teeth no teeth anterior to them in the upper jaw; cheek teeth employed in chewing cud 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 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)  

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larger and longer in order to deal with tough shearing problems Ungulates & some herbivores o for macerating vegetation o 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 Selenodont

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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 line o 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

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Humans, rhinos, some primitive ruminants, rodents

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Epidermal Teeth  Keratinized (horny) teeth in living agnathans’ buccal cavity and tongue, for rasping  Anuran tadpoles: temporary lips perched on poorly developed jaws, for rasping algae and other vegetation; shed and replaced by bony ones at metamorphosis

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Turtles, crocodilians, Sphenodon, birds, monotremes: temporary horny egg tooth used to crack egg shell Platypus: horny teeth replace bony teeth and remain throughout life Horny beaks of turtles and modern birds have serrations that perform some of the functions of teeth

PHARYNX  Part of the digestive tract that had pharyngeal pouches in the embryo  Opens into the esophagus  Fishes: functional part of the respiratory system  Most constant features: o Glottis – slit opening into larynx o Openings of paired auditory tubes – leads to the middle ear cavity o 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 1 st 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 wall o Laryngeal pharynx dorsal to the larynx in mammals in which the opening to the esophagus is caudal to the glottis o Lingual tonsils develop on the tongue near its attachment to the hyoid bone o Tonsils as lymphoid masses that serve as the body’s first line of defense against infective agents o 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 

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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 WALL  4 histological layers (outward): mucosa, submucosa, muscularis externa, serosa o Differences lie in the thickness and the nature of glands  Mucosa 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 externa o 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  Serosa o Consists of loose connective tissue (adventitia) + covering of visceral peritoneum o Exudes small amounts of serous fluid that lubricates surface of viscera, reducing friction o Peritonitis: inflammation of serosa, leads to exudation of excessive quantities of fluid o 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 ciliated  Many Teleosts: cilia in the stomach  Some adult amphibians: cilia in the oral cavity, pharynx, esophagus, & stomach  Cilia are present for a time in the stomach of human fetus  Peristalsis chiefly responsible for moving foodstuffs along alimentary canal