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LIVER, GALL BLADDER

LIVER

Where the stomach meets the small intestine in tunicates, there are a number of tubules collectively called the pyloric gland which empty through a duct into the intestine. The cells resemble vertebrate liver cells (especially those of the most primitive fish, the hagfish) and function in the storage of glycogen and the removal of foreign substances from the blood, as in vertebrate livers (Burighel, from Harrison, 1997, p. 257). In all chordates, the veins which reach this structure from the intestines are portal veins (Stach, 2000). In lancelets, the brownish-green hepatic sacculations (also called the hepatic cecum) are the primary source of digestive enzymes, releasing protease, amylase, and lipase. (Ruppert, from Harrison, 1997, p. 444-5). Amphioxus possesses a hepatic portal system in which blood from intestines brought to a capillary network in liver before the hepatic vein returns it to general circulation. The same pattern of circulation exists in vertebrates (Willey, 54).

AMPHIOXUS

In hagfish, the liver no longer functions in enzyme production and food absorption as in Amphioxus (Romer, p. 390). Gnathostome livers increase in size and are lobed (Weichert, 1970, p. 194).

HAGFISH

LAMPREY

SHARK

GAR

BOWFIN

PERCH

LUNGFISH

FROG

SALAMANDER

TURTLE

OPOSSUM

CAT

SHEEP

PIG

MONKEY

HUMAN MODEL

GALL BLADDER

In most vertebrates, the bile produced in the liver can be concentrated and stored in the gall bladder. The cystic duct from the gall bladder and the hepatic duct from the liver fuse to empty into the common bile duct which, in turn, empties into the small intestine. Jawless fish possess both a liver and gall bladder although the liver is small. Lamprey gall bladders and bile ducts degenerate in adults (Weichert, 1970, p. 194). A few species of shark, many birds, and a number of mammals (some artiodactyls, rodents, whales, seals, and perissodactyls) lack gall bladders (Weichert, 1970, p. 194). Although pigeons lack a gall bladder, it can be present during embryonic development (Weichert, 1970, p. 194).