1. Digestive System and Derivatives
Derivatives include respiratory system, liver, pancreas, gall bladder and endocrine structures
All are endodermal in origin
Digestive System includes digestive tract
Mouth & Pharynx — Small Intestine
Esophagus — Large Intestine
Stomach — Cloaca (or derivative)
Also includes associated digestive glands: liver, pancreas and gall bladder

2. Fig 13.1 – Digestive tract components
Figure 13.1
Fig 13.1 – Digestive tract components

3. Embryonic Origin of Digestive Tube
Embryonic Origin of Digestive Tube by 2 Basic Methods
Cyclostomes, Actinopterygians, and Amphibians = gastrulation provides a “tube-within-a-tube” arrangement. Inner tube is endodermally derived and becomes gut. 
All other vertebrates have:
The epiblast oriented on top of the hypoblast in flat sheets. The hypoblast is continuous peripherally with the endoderm of the prospective yolk sac.
Development of head, lateral body, and tail folds separate the embryo from extraembryonic membranes.
The endoderm folds upon itself to form a tube continuous ventrally with the yolk sac  forms the gut.

4. Development of Openings to Gut Tube
Protostomes = blastopore forms the mouth; the anus is derived secondarily
Includes Annelids, Molluscs, and Arthropods
Deuterostomes = blastopore becomes the anus; the mouth forms later as an independent perforation of the body wall
Includes Echinoderms and Chordates
In vertebrate development the head turns downward over the surface of the yolk, forming an ectodermal pocket (stomodeum) which represents the primitive mouth cavity

5. Development of Openings to Gut Tube
Stomodeum is separated from the pharyngeal region of the gut by a membrane (pharyngeal membrane) that eventually breaks down so that the oral cavity and pharynx become continuous.
Proctodeum is similar invagination at the posterior end of the gut, separated from the gut by the cloacal membrane that eventually disappears, leaving a tube open at both ends. 
The mouth and teeth are derived from ectoderm.

6. Fig 13.2 – Embryonic formation of the digestive system
Figure 13.2
Fig 13.2 – Embryonic formation of the digestive system
Early amniote embryo
Generalized amniote embryo
Ventral view of isolated gut
Lateral view of differentiating gut

7. Development of Openings to Gut Tube
The boundary of the mouth ideally is the junction of the stomodeum (ectodermal) with the pharynx (endodermal).
In practice, definite anterior and posterior limits to the mouth are difficult to establish, and differ among vertebrate groups.
Landmarks used in distinction as markers of the mouth (ectodermally derived) include:
Nasal Pits (= nasal placodes)
Rathke’s Pouch (= hypophyseal pouch)
Evolutionary trend: toward inclusion of more ectoderm inside the mouth in advanced forms
Primitively, stomodeal structures are forced outside the mouth through differential growth

8. Fig 13.4 – boundaries of the mouth cavity
Figure 13.4
Fig 13.4 – boundaries of the mouth cavity

9. Mouth Cavity
Lined by skin, includes teeth and salivary glands as components
Teeth are homologous with the integument of some fishes and placoid scales (denticles) of shark skin
 Location of teeth
Fish = found on palate (roof of mouth), margins of jaw, gill arches
Amphibians/Reptiles = found on some bones of the palate and margins of maxillary, premaxillary and dentary bones
Mammals = found only on margins of maxillary, premaxillary and dentary bones

10. Mouth Cavity
Evolutionary trend in mammals = reduction in numbers of teeth from primitive to advanced mammals
Primitive mammal number is 44 (humans with 32)
Whales have an increased number as a specialization to their very large mouth
Birds have no teeth, except for primitive Mesozoic forms (associated with reduced weight for flight)
Turtles also lack teeth; instead have a hard, keratinized beak
Number of generations of teeth is reduced from primitive (continuous replacement) to advanced (1 or 2 sets) vertebrates

11. Degree of Tooth Differentiation
Homodontous Condition = all teeth are similar, generally conical in shape
Most vertebrates
Heterodontous Condition = specialization of teeth
Typical state for a few reptiles, Therapsids, and Mammals
Teeth include:
Incisors (front) - used for cropping
Canines - behind the incisors, used for tearing
Molars (cheek teeth) - furthest back in mouth, used for chewing
Teeth in heterodontous vertebrates are used for capture or cropping of food and chewing
Chewing aids in digestion by increasing surface area of food available for digestion
This increases digestive efficiency and provides energy necessary to support high rates of metabolism of mammals

12. Homodontous Teeth from salamander
Heterodontous Teeth from fox

13. Salivary Glands Formed from invaginations of the mouth lining
Mucous Glands = produce mucous; lubrication of food
Serous Glands = watery secretion containing enzymes; initiates digestion of carbohydrates (salivary amylase)
Mixed Glands = mucous and serous secretions
Snake venom glands are modified serous salivary glands

14. Fig 13.37 – Salivary glands in a dog

15. Fig 13. 35 – Oral glands of reptiles
Fig – Oral glands of reptiles. Venom glands derived from Duvernoy’s gland.

16. Palate Forms roof of mouth
Composed of bone, lined by epithelium and connective tissue
Fish, Amphibians and Birds have only a primary palate present
Crocodilians and mammals also have a secondary palate, which allows simultaneous chewing and breathing in mammals, and breathing while mouth is submerged in crocodiles
Secondary palate separates nasal passages from mouth

17. Fig 7.57 – Primary and Secondary palates in vertebrates

18. Pharynx
Shared region between digestive and respiratory systems – Respiratory system represents a derivative of the digestive tract.
Other pharyngeal derivatives
Thyroid - present in all vertebrates, always derived as outpocketing from floor of 1st pharyngeal pouch
Fish = thyroid tissue becomes dispersed along the ventral aorta in adults
Tetrapods = remains as a single or bilobed gland
Function = produces Thyroid Hormones that increase metabolic rate and regulate early development and growth
C-cells are also present (only in mammals); produce Calcitonin which decreases blood calcium levels by reducing bone resorption

19. Other Pharyngeal Derivatives
Parathyroids - not present in fishes; present in all tetrapods
Amphibians and Reptiles = derived from ventral regions of pouches 2-4
Birds = from ventral regions of pouches 3-4
Mammals = from dorsal regions of pouches 3-.
Secrete parathyroid hormone which increases blood calcium levels by promoting bone resorption

20. Other Pharyngeal Derivatives
Thymus - found in all vertebrates except Cyclostomes
Derived from various pouches in the different vertebrate groups
 Function: immunological role, production of T-lymphocytes  cell-mediated immunity
Ultimobranchial Bodies = derivatives of ventral part of 5th pharyngeal pouch in all vertebrates except mammals
Secrete Calcitonin, so they are presumably homologous with C-cells of mammalian thyroid gland
1st Pharyngeal Pouch forms spiracle in Elasmobranchs
Forms the tympanic cavity and Eustachian tubes in Tetrapods

21. Comparative
Pharyngeal Pouch
Derivatives in
Vertebrates

22. Digestive Tube Proper
General Sequence: anterior to posterior is Esophagus  Stomach  Intestine  Cloaca (or anus)
Esophagus:
Function = food transport; secretes mucus to aid passage
Birds show specialized Crop = sac-like structure adapted for food storage

23. Stomach
None present in Cyclostomes, chimeras, lungfish, and some teleosts (primitive condition)
When present, functions in food storage, physical treatment of food, initiates digestion
Food storage is the primary function (and probably the original evolutionary function)
Physical treatment evolved somewhat later as food is taken in large chunks
Digestion probably is latest function to evolve

24. Stomach Birds and Crocodiles
Muscular tissue of stomach is concentrated posteriorly as a gizzard
Anterior stomach is glandular (Proventriculus)
Because birds lack teeth, many will swallow small pebbles (grit) that lodge in the gizzard and aid in grinding food
Functional analog to teeth in mammals

25. 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
The rumen, reticulum, and omasum are derived as modifications of esophagus; abomasum is the true stomach
Ruminant-like digestion occurs in one bird, the Hoatzin
Folivorous (eats leaves) bird with foregut fermentation similar to ruminant digestion
Enlarged crop & lower esophagus house symbiotic bacteria

26. Fig 13.42 – Ruminant digestion in the bovine stomach

27. Foregut fermentation in Hoatzin digestive system

28. Intestine Majority of digestion and absorption occurs here
 Sharks and some other fishes have a spiral intestine = cigar-shaped body with spiral valve internally
Greatly increases surface area for absorption
Increased surface area in Tetrapods is by elongation and coiling of intestines along with folding of internal surfaces
Intestine is longer in herbivores than in carnivores because plant matter is more difficult to digest

29. Intestine
Evolutionary Trend in intestine structure = increased intestinal surface area (primitive  advanced) associated with higher metabolic rates in advanced vertebrates
Hagfish lack spiral valve; poorly developed in lampreys
Spiral valve is present in sharks and some other fishes
Elongation and coiling with internal folding in Tetrapods

30. Fig 13.27 – Stomach and Intestines in non-mammalian vertebrates

31. Fig 13.28 – Stomach and Intestines in various mammals
Figure 13.28
Fig – Stomach and Intestines in various mammals

32. Fig 13. 29 – Digestive tracts of various fishes
Fig – Digestive tracts of various fishes. Note spiral valves in several species and elongation of intestine in perch