OBL REFERENCES
GASTROINTESTINAL TRACT

ORGANS OF THE GASTROINTESTINAL TRACT

    There are similarities of the gastrointestinal tract shared by chordates.   Food enters the body through the oral cavity (mouth), as in that of a lancelet pictured below.

LANCELET

PHARYNX

     From the mouth, food passes through the pharynx to the intestine.   The ability to pass water through pharyngeal slits is involved in feeding in primitive filter-feeding chordates and hemichordates: food from the water becomes stuck in the mucus lining the pharynx and is carried to the intestine by ciliary action. 

CAT

CAT

PHARYNX

ESOPHAGUS

     In chordates and vertebrates, food travels from the pharynx into the esophagus where peristalsis moves it through the remainder of the gastrointestinal tract (Burighel, from Harrison, 1997, p. 255)

HAGFISH

HAGFISH

HAGFISH

SHARK

SHARK

 

GAR

GAR

PERCH

PERCH

     The esophagus became more prominent in tetrapods than in fish (Romer, p. 378).  The primitive condition of esophageal cilia still persists in some fish, amphibians, and reptiles (Stevens; Weichert, 1970, p. 178).  Reptiles have a much longer esophagus in contrast to the short organs observed in fish and amphibians.  Reptiles are the first group in which there is a clear distinction between the stomach and esophagus (Weichert, 1970, p. 181).

PERCH

PERCH

FROG

FROG

FROG

 

ALLIGATOR

ALLIGATOR

CHICKENCHICKEN CHICKEN

 

CHICKEN

OPOSSUM

OPOSSUM

CAT

CAT

PIG

ESOPHAGUS

SHEEPSHEEP

GOAT

GOAT

MONKEY

MONKEY

MONKEY

STOMACH

     In filter feeders, food-bearing mucus is transported to an acidic area of the gastrointestinal tract where the mucus becomes less viscous and the food is released (Hoar, 1983, p. 420).  Stomachs evolved in the ancestors of gnathostomes.  In jawless fish, the stomach is virtually nonexistent, represented by a small enlargement of the distal end of the esophagus. (Weichert, 1970, p. 180).  In gnathostomes (and perhaps late fossil jawless fish such as thelodonts), part of the GI tract develops into a stomach.

THEOLDONT

  In gnathostomes, the stomach can be divided into several regions.  There is a body where the food is mechanically and chemically digested, a fundus where food can be temporarily stored, and a narrow pyloris region which is separated from the small intestine by the pyloric sphincter.   The glandular cardiac region of the stomach is shared by tetrapods (Stevens, p.16).  Mammals have separate cells for the secretion of pepsin and hydrochloric acid while these tasks are performed by the same cells in other vertebrates (Webster, 1974, p. 345). 

   

SHARK

SHARK

SHARK
STOMACH

GAR

GAR

BOWFINBOWFIN

FROG

FROG

 

SALAMANDER

SALAMANDER

 

TURTLE

TURTLE

ALLIGATOR

ALLIGATOR

CHICKEN

CHICKEN

 

OPOSSUM

OPOSSUM

 

CAT

CAT

SHEEP

SHEEP

GOAT

GOAT

COW

COW

PIG

PIG STOMACH

MONKEY

STOMACH

MONKEY

HUMAN MODEL

HUMAN MODEL

HUMAN MODEL