Comparative Vertebrate Anatomy

Evolution of the Digestive System

           
The Coelem is the body cavity.  It is seperated most simply into the Thoracic Cavity and the Abdominopelvic Cavity.  The Thoracic Cavity holds the heart, lungs, esophagus, and trachea.  It is seperated from the Abdominopelvic Cavity by the diaphragm.   The abdominopelvic cavity contains the stomach, liver,  intestines, and colon.
The digestive system consists of the alimentary canal, which is essentially a tube that extends from the mouth to the anus.  Within this tube are specialized formations that we know as lips, teeth, tongue, pharynx, esophagus, stomach, small intestines, large intestine, and lying outside of the alimentary canal are the digestive glands (salivary gland, liver, and pancreas).  Digestion refers to both mechanical and chemical activities.  Mechanical digestion involves the tearing, grinding and mixing of food with enzymes.  Chemical digestion is done by various digestive enzymes and other chemicals such as gastric acid. There is quite a bit of variation in the oral cavity and pharynx across the different classes of vertebrates.  The emergence of the palate, which separates the nasal cavity from the oral cavity, differentiated terrestrial vertebrates from their aquatic ancestors.   In birds and most reptiles there are a pair of folds running through the roof of the oral cavity forming an air passage from the nares to the pharynx.  Only crocodilians and mammals show a complete separation between the oral and nasal cavities.  In mammals there is a posterior soft fleshy palate that supplements the anterior hard bony palate.  In fishes and aquatic amphibians, the pharyngeal arches gave rise to gills.  In terrestrial vertebrates the gills are lost and the pharynx is used as a passage between the mouth and the esophagus/trachea. 

Almost all Vertebrates have a stomach.  Exceptions to this rule are some cyclostomes, lungfishes, some bony fishes.  The intestine varies in size among vertebrates. Animals have different diets and live in different environments.  These things can be observed in the size and specializations within the intestines of the different animals.  In chondrycthyes we see a shorter specialized valvular intestine.  The valvular intestine slows the movement of food through the shorter intestine allowing for more nutrient absorption and water re-absorption.  In terrestrial vertebrates there is a tendency towards a longer intestine to assist with nutrient absorption and water re-absorption.  An increase in length allows the intestine to extract more nutrients and water from the food passing through it.  The intestine also has villi, fingerlike outgrowths, that cover the entire surface of the mucosa and the villi are covered in closely packed projections called microvilli.  These structures help to increase the surface area of the intestine, which thereby increases the level of absorbtion that the intestine is capable of. 

The final destination of the digestive tract is the cloaca in elasmobranch fish, birds, amphibians, and most reptiles.  Most ray finned fish as well as mammalian adults have a rectum companied with separate urinary and reproductive tubes that have their own opening.  Monotremes (egg laying mammals)  retain the cloaca as do marsupials although in a much reduced form.  The phylogeny of the cloaca can be seen through the ontogeny of mammals as the placental mammalian embryo do have a cloaca for a short period of time.