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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.

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. Pharyngeal slits became involved with respiration secondarily and higher vertebrates use this mucociliary mechanism as a way of directing microbes and dirt to the stomach and away from the lungs. Most filter feeding animals make use of mucus to capture the food and cilia to transport it. The development of a perforated, muscular pharynx in chordates allowed the change from microphagy (feeding on small material suspended in water) to macrophagy (feeding on larger material)(Barrington).

CAT

ESOPHAGUS

In hemichordates, 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

SHARK

GAR

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). In birds the esophagus forms a separate sac known as the crop. A few birds energy rich secretions from the crop known as “crop milk” to nourish hatchlings (Webster, 1974).

The muscle of the esophagus in fish is striated in contrast to the majority of tetrapods in which the esophagus is lined by smooth muscle. The amount of smooth and striated muscle found in the esophagus varies in different groups of mammals (Stevens). Ruminants possess skeletal muscle down their entire esophagus, giving them a greater voluntary control over its action (as evidenced in their ability to regurgitate swallowed food, the cud, to continue chewing it). In some fish, amphibians, and a bat (Plecotus auritus) the esophogus is lined with columnar epithelia which secretes pepsin and/or HCl. Sometimes this condition is even observed in humans (Stevens). The crossing of the passageways for air and food makes an animal vulnerable to asphyxiation. In whales, there is no pharyngeal chiasma since the larynx wraps around the pharynx reach the nostrils (Webster, 1974).

PERCH

FROG

ALLIGATOR

CHICKEN

OPOSSUM

CAT

PIG

SHEEP

GOAT

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.

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).

In the two monotremes, the stomach is lined with stratified squamous epithelia (Stevens). Some insectivores possess a stomach lined by keratinized epithelium to protect them from hard insect skeletons and release their gastric secretions from one duct rather then through numerous gastric pits. Ruminants possess a four chambered stomach which contains the symbiotic microbes which help them digest plant material and can hold up to 60 gallons of material (Webster, 1974). The stomachs of ruminant artiodactyls, whales, and hippos are also divided into compartments (Weichert, 1970, p. 183). There is considerable stomach variation in bats; in vampire bats the stomach forms a long tube. The bat Desmodus possesses a pouch at the distal end of its stomach which holds the ingested blood (Weichert, 1970, p. 183). Kangaroos also possess a long, tubular stomach (Stevens).

SHARK

GAR

BOWFIN