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Digestion
Digestive Systems
 Absorb nutrients necessary to
maintain life, eliminate wastes
 Ingestion
 Digestion
 Absorption
 Elimination
 Digestion—process by which food is
chemically and mechanically broken
down for use by the body
Digestive Systems
 No digestive system
 Sponges, tapeworms
 Absorb only small molecules
 Digestion intracellular
Digestive Systems
 Incomplete
 Cnidarians, planarians
 Single body opening
(Gastrovascular cavity)
 Limited enzymes
 Digestion begins
extracellularly
 Digestion completed
intracellularly
 No regions w/ specialized
function
Digestive Systems
 Complete
 Annelids, mollusks,
arthropods, vertebrates,
etc. (most animals)
 Two openings (mouth &
anus)
 Different regions w/
specialized function
 Digestion primarily
extracellular
Digestive Systems
 Continuous feeders
 Filter feeders (clams, mussels,
barnacles, etc.)
 Must complete digestion quickly
 Discontinuous feeders
 Most animals
 Feed periodically
 Need storage area in gut
Complete Digestive System
 Functions
 Mechanical processing & motility
 Secretion of digestive enzymes
 Chemical digestion
 Absorption
 Elimination of wastes
 Anatomy determined by lifestyle
 Carnivore
 Herbivore
 Omnivore
Mouth
 Initial digestion begins
 Mechanical break-down of food
 No teeth
 Worms, birds—swallow food whole

Grinding done lower in digestive tract
 Insects—some chew, some suck
 Arachnids—no chewing


Suck existing fluids
Inject digestive fluids into prey
Mouth
 Teeth
 Vertebrates only
 Incisors—shearing
 Canines—puncture, tearing


Capture prey
Fighting predators or within
species
 Premolars—grinding
 Molars—crushing
Mouth
 Herbivores
 Plant material tough
 Emphasis on grinding,
crushing
 Broad, flat premolars &
molars
 Limited or absent canines
Mouth
 Carnivores
 Animal material easier
to mechanically tear
 Emphasis on piercing,
cutting
 Long, pointed canines
 Molars & premolars
modified for shearing
or piercing
Mouth
 Omnivores
 Combination teeth
 Varies based on amount of
each type of food
Mouth
 Salivary glands
 Secrete via ducts into mouth
 Lubricate & moisten food
 Salivary amylase

Begins to digest starches
 Tongue
 Grab food
 Manipulate food
 Taste
Esophagus
 Muscular tube
 Lined by stratified
squamous epithelium
 Moves food from mouth to
next step
 Peristalsis—rhythmic
contractions of
gastrointestinal tract
Upper GI
 Crop
 Insects
 Annelids
 Most birds
 Temporary storage
Upper GI
 Proventriculus
 Birds, many invertebrates
 Secretes digestive enzymes
 Gizzard
 Birds, many invertebrates,
some reptiles
 Dinosaurs
 Thick, muscular walls
 Grinding food


Chitinous plates in some insects
& mollusks
Birds swallow “grit” to use for
grinding
Upper GI
 Stomach (monogastric)
 Temporary storage
 Mechanical churning
 Digestive enzymes
 Combined functions of
crop, proventriculus,
gizzard
 Pepsin—breaks down
proteins
 Hydrochloric acid—slows
salivary amylase, activates
pepsin
Upper GI
 Stomach (ruminant)
 Cattle, goats, llamas, camels,
deer, sheep, etc.
 Four-chambered stomach

Rumen, reticulum, omasum,
abomasum
 Food swallowed, separated in




reticulorumen
Solid (cud) regurgitated from
reticulum
Cud chewed for further
breakdown, then swallowed
Moved to omasum, water &
inorganic minerals absorbed
Moves to abomasum, like
monogastric stomach
Lower GI
 Small intestine
 Short in carnivores
 Long in herbivores
 Duodenum
 Jejunum
 Ileum
Lower GI
 Secretion of enzymes
 Intestinal mucosa
 Gall bladder
 Pancreas
 Absorption
 Villi—folds of intestinal
lining
 Microvilli—folds of intestinal
cells
Lower GI
 Cecum
 Single in mammals
 Double in most birds & some
reptiles
 Large in herbivores
 Reduced in carnivores
 Remove fluids & salts
 In herbivores has large
numbers of bacteria to break
down plant material
Lower GI
 Colon (large intestine)
 Most food digested & absorbed
by this point
 Water & waste
 Functions to reabsorb water
 Diarrhea—water not absorbed
properly
 Rectum
 Storage of feces ready for
elimination
 Anus
 Muscular sphincter controlling
elimination of feces
Other Organs
 Liver
 Secretes bile (breaks down fats)
 Many other important functions
 Gall bladder
 Stores & concentrates bile
 Not present in all animals, even
if closely related



Absent horse, deer; present cow,
goat, sheep
Absent doves; present hawks &
owls
Absent rats; present mice
 Pancreas
 Many enzymes to break down
food
 Buffers against HCl from
stomach