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THE PERICARDIUM

     The pericardium is the serous membrane which surrounds the heart.  Pericardial fluid is secreted into the pericardial cavity (between the parietal and visceral layers of the pericardium) to reduce friction as the heart contracts. 

     In hemichordates, the heart is surrounded by a pericardium formed by a blind sac of the coelom, although it doesn’t completely separate the heart from the coelom (Benito, form Harrison 1997, p. 20; Hickman).  Tunicates possess both a pericardium and pericardial fluid that contribute to the pressures which move blood (Webster, 1974, p. 47).  Bivalves, sharks and lungfish have a rigid pericardium which helps to create a negative pressure to facilitate the filling of atria (Hoar, 1983)

     In hagfish, the pericardium is a continuation of the peritoneum that lines the abdominal cavity (Guenther, 151).  The lamprey heart is completely enclosed in the pericardial cavity (Kardong).  In larval lampreys, the pericardial cavity is connected to the coelomic cavity (as in hagfish) but the two cavities are separated in adulthood (Weichert, 1970).  A pericardial cavity separate from the abdominal cavity is a characteristic of vertebrates.

HAGFISH HAGFISH HAGFISH

SHARK

SHARK HEART

SHARK HEART
GARGAR HEART

BOWFIN

BOWFIN

LUNGFISH

LUNGFISH

LUNGFISH

FROG

FROG

FROG

OPOSSUM

OPOSSUM

OPOSSUM

CAT

CAT

MONKEY

MONKEY