Hemichordate intestines possess goblet cells and microvilli forming a brush border extending from tall epithelial cells (Benito, form Harrison 1997, p. 88).  While the intestinal wall of urochordates is typically smooth, a few species possess grooves.  Intestinal cells include absorptive cells, endocrine cells (whose secretions include gastrin, pancreatic polypeptide, and secretin), and mucus cells. There is very little muscle along the GI tract and most material is moved through ciliary action (Burighel, from Harrison, 1997, p. 256)  In lancelets, cells of the stomach and intestine are ciliated. Endocrine cells exist in the intestine and a muscular sphincter surrounds the anus (Ruppert, from Harrison, 1997, p. 444-5; Burighel, from Harrison, 1997, p. 255).  In lancelets, cilia move food to the intestine where flagella mix with digestive enzymes produced primarily from the hepatic cecum (Ruppert, from Harrison, 1997, p. 354). 

     Unlike hemichordates and protostomes, the digestive tract of chordates does not proceed to the tip of the abdomen.  One of the major traits which all chordates share is that of a muscular, postanal tail.

LANCELET

LAMPREY

ZEBRAFISH EMBRYO

     Fish increase the surface area of the intestine through a fold which runs along the intestine (the typosole in jawless fish and the spiral valve in cartilaginous and primitive bony fish).  Placoderms also possessed a spiral valve in the intestine. (Weichert, 1970, p. 187)

Some teleosts have increased the length of their intestines while others possess many blind pockets near the proximal end of the intestine called pyloric caeca (mackerel may have 200 of these structures) (Webster, 1974).  Most fish lack intestinal glands which extend into the submucosa (Stevens). Tetrapods increase the surface area of their small intestine through finger-like projections known as villi.  Tetrapods possess lymphatic capillaries known as lacteals in these villi and possess glandular crypts of Lieberkuhn in their intestines. (Stevens, p.18).  The villi of a frog’s intestine is pictured below.

     In amphibians, there is an ileocecal valve separating the small and large intestines (Romer, p. 388) and the proximal portion of the small intestine becomes the specialized duodenum (Kardong, p. 512).   Tetrapods lose the intestinal spiral valve which is present in gnathostome fish (Romer, p. 387).  In about 2% of humans, a portion of the yolk stalk is retained as the Meckel’s diverticulum from the ileum.  Although this pouch is usually about 2 inches, it can reach 7 inches (and may even open through the navel, requiring surgery). (Weichert, 1970, p. 189). Human intestines can vary between 11 to almost 26 feet in length (Weichert, 1970, p. 186). 

     Intestinal sodium transport similar to that which occurs in vertebrates is known in most coelomates (Stevens, p. 296).   In mammals, intestinal villi become more prominent (Romer, p. 386). Higher apes developed an appendix (Ankel-Simons, p. 385).  Some edentates possess 2 ceca (Weichert, 1970, p. 189)   Some insectivores and one rodent (the mountain beaver Aplodontia rufa) possess a cloaca (Stevens). 

    In amniotes, a blind pouch known as the cecum exists where the small intestine joins the large intestine (Romer, p. 388).  There are a number of variations of the cecum known in mammals.  The cecum varies in the armadillo genus Dasypus from being absent to the presence of two small ceca (Stevens).  In viverrids (carnivores), the cecum may be present, vestigial, or absent (Stevens, p. 58).  Many marsupials lack a cecum (Stevens).

HAGFISH

LAMPREY LARVA

LAMPREY

SHARK

BOWFIN

PERCH

LUNGFISH

SALAMANDER

FROG

TURTLE

ALLIGATOR

CHICKEN

OPOSSUM

CAT

SHEEP

GOAT

COW

PIG

MONKEY

HUMAN MODEL