406 



COLLEGE ZOOLOGY 



secrete digestive ferments or enzymes (p. 220) and hydro- 

 chloric acid; these help dissolve the food so that it can be 

 absorbed. A circular muscle, called the pyloric sphincter, regu- 

 lates the passage of food into the small intestine. 



Connected with the small intestine by a bile duct is the liver. 

 This organ secretes an alkaline fluid called bile which is poured 

 into the intestine, where it divides fatty food into particles fine 

 enough to penetrate the walls of the intestine. Often an en- 

 largement, the gall-bladder, is present, in which the bile is stored. 

 The liver also changes sugar into a substance called glycogen, 

 which is stored up as a reserve for the future needs of the animal. 



Another large gland, the pancreas, secretes a digestive fluid, 

 the pancreatic juice, which enters the intestine through the 

 pancreatic duct. This fluid contains three important ferments; 

 (1) amylopsin, which forms soluble sugar from starch, (2) trypsin, 

 which converts proteid into peptones, and (3) steapsin, which 

 changes fat into soluble fatty acids and glycerin. 



The intestine is usually longer than the body and therefore 

 coiled within the abdomen. Through its walls most of the di- 

 gested food is absorbed into lymphatic tubes and blood capil- 

 laries. The absorbent surface is often increased by folds and 

 small prominences called villi. Undigested particles are formed 

 into faces in the posterior part of the intestine and ejected 

 through the anus. In many vertebrates the intestine opens 

 into a terminal sac, the cloaca, into which the excretory and 

 reproductive ducts also open. 



Circulatory System. — The blood into which the digested 

 food passes from the alimentary canal consists of a colorless 

 plasma containing passive red corpuscles and active, ameboid, 

 colorless corpuscles. The color of the red corpuscles is due to 

 the presence of a substance called hcemoglobin. The heart of I 

 vertebrates lies in a part of the ccelom termed the pericardium. 

 It consists of at least two chambers: (1) an auricle into whicl 

 the blood is brought by the veins, and (2) a ventricle which forces: 

 the blood through the arteries. 



