382 TEXTBOOK OF ZOOLOGY 



In animals without a skeleton muscle bands are arranged in both 

 circular and longitudinal directions. The contraction of the circular 

 group tends to lengthen the body, and the shortening of the longi- 

 tudinal strands draws the body along. The pressure exerted on the 

 coelomic fluid is thought to be a factor in bringing about an even 

 extension of the body by this means. In echinoderms with the water 

 vascular system the pressure is exerted on water in a system of tubes 

 which extend to make contact with the surface over which the animal 

 is moving. 



The Dig"estive System. — The digestive system is typically a 

 straight tube extending through the length of the trunk of primitive 

 vertebrates. In the higher forms there are many outgrowths, such 

 as digestive glands and respiratory organs. The anterior region of 

 the digestive tube is the mouth cavity which contains teeth on the 

 jaws, a tongue, and receives saliva from salivary glands. Following 

 the mouth is the pharynx or throat region which receives the internal 

 nostrils, the Eustachian tubes from the middle ears and opens into the 

 esophagus posteriorly. The esophagus is usually tubular and propels 

 the ''swallows" of food posteriorly by consecutive waves of contrac- 

 tion, a process known as peristalsis. It leads to the saclike stomach, 

 whose walls possess gastric glands for secretion of a digestive fluid 

 containing enzymes (ferments) and weak hydrochloric acid. The 

 peristaltic contractions continue along the wall of the stomach to help 

 digestion by churning and mixing the food with digestive juices. 

 At the posterior end a pyloric valve in the form of a sphincter 

 muscle guards the entrance to the small intestine which follows. This 

 is a convoluted tube in most of the advanced forms of vertebrates 

 and is divided into the anterior duodenum, middle jejunum, and 

 posterior ileum. It is usually longer than the body and therefore 

 it is coiled. Its walls produce digestive enzymes from glands and 

 it receives digestive juices from two other glands : the liver and 

 the pancreas. The small intestine serves both as a digestive organ 

 and as the principal absorptive organ of the body. Its internal 

 lining is provided with numerous fine fingerlike projections which 

 increase the inner surface and enhance absorption. The digested 

 food is taken up by the lymphatic spaces and by blood vessels which 

 are embedded in the wall just outside of the lining epithelium. The 

 liver is the largest organ in the body of most vertebrates. It 

 secretes the bile which is stored in the thin-walled gall Madder, 



