FISHES 131 



several times, thus forming a coil, the posterior end of which 

 finally opens to the exterior just in front of the anal fin. 



In the inner lining of the stomach and intestine are special 

 cells which make up digestive glands. These have the power 

 to manufacture digestive ferments (P. B., 53), which are 

 forced out into the alimentary canal when food is present. 

 As in plants, these ferments dissolve the foods and make them 

 ready for use in the body. In addition to the digestive 

 glands in the lining of the alimentary canal there are glands 

 outside the digestive tube. One of these is the liver (Fig. 98), 

 which secretes bile. This is carried to the intestines by a 

 tube called the bile duct. In the liver is a sac (bile sac) 

 (Fig. 98) which holds any excess of bile. When the food has 

 been digested it is absorbed by thin-walled blood vessels 

 found in the lining of the alimentary canal, and so passes 

 into the blood to be distributed around the body. 



99. Blood and circulation. — Instead of ducts and sieve 

 tubes (P. B., Figs. 14, 15, 16) as in the seed plants we studied, 

 the fish has blood vessels to distribute digested foods to various 

 parts of the body. In addition to these the fish possesses a 

 heart (Fig. 99), which aids in pumping or forcing the blood 

 through blood vessels, thus keeping it in constant motion. 

 The blood vessels are of three kinds ; namely, arteries, capil- 

 laries, and vein's. The arteries have muscular and elastic 

 walls which contract and so aid the heart in forcing the blood 

 along its course. The arteries always carry the blood away 

 from the heart, and they subdivide into smaller and smaller 

 tubes. At the ends of the smallest arteries are tiny, short, 

 thin-walled blood vessels, known as capillaries. Capillaries 

 permit the digested food to osmose through their walls into 

 the adjacent cells, and, in turn, absorb waste matters from 

 the cells. 



