. Essentials of biology presented in problems. Biology. 278 THE VERTEBRATE ANIMALS Digestive System. — The gullet leads directly into a baglike stomach. There are no salivary glands in the fishes. There is, however, a large Uver, which appears to be used as a digestive gland. This organ, because of the oil it contains, is in some fishes, as the cod, of considerable economic importance. Many fishes have outgrowths like a series of pockets from the intestine. These structures, called the pyloric cceca, are believed to secrete a digestive fluid. The intestine ends at the vent, which is usually lo
. Essentials of biology presented in problems. Biology. 278 THE VERTEBRATE ANIMALS Digestive System. — The gullet leads directly into a baglike stomach. There are no salivary glands in the fishes. There is, however, a large Uver, which appears to be used as a digestive gland. This organ, because of the oil it contains, is in some fishes, as the cod, of considerable economic importance. Many fishes have outgrowths like a series of pockets from the intestine. These structures, called the pyloric cceca, are believed to secrete a digestive fluid. The intestine ends at the vent, which is usually located on the ventral side of the fish, immediately in front of the anal Anatomy of the carp : br, branchiae, or gills; c, heart; bladder; ci, intestine. /, liver; vn, swimming Swim Bladder. — An organ of unusual signifieanee, called the swim bladder, occupies the region just dorsal to the food tube. In young fishes of many species this is connected by a tube with the anterior end of the digestive tract. In some forms this tube persists throughout life, but in other fish it becomes closed, a thin, fibrous cord taking its place. The swim bladder aids in giving the fish nearly the same weight as the water it displaces, thus buoying it up. The walls of the organ are richly supplied with blood vessels, and it thus undoubtedly serves as an organ for supplying oxygen to the blood when aU other sources fail. In some fish (the dipnoi, p. 284) it has come to be used as a lung. Circulation of the Blood. —- In the vertebrate animals the blood is said to circulate in the body, because it passes through a more or less closed system of tubes in its course around the body. In the fishes the heart is a two-chambered muscular organ, a thin-walled auricle, the receiving chamber, leading into a thick-walled muscular ventricle from which the blood is forced out. The blood is pumped from the heart to the gills; there it loses some of its carbon dioxide; it then passes on to other parts.
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