Fig. 16-8. Planaria, diagrammatic 

 cross section. 



GASTROVASCULAR CAVITY 



The Digestive System - 295 



ENDODERMAL EPITHELIUM 



EPITHELIUM 



systems. The intercellular spaces of the meso- 

 derm are filled with a body fluid, which dis- 

 tributes food substances absorbed from the 

 numerous branches of the gastrovascular cav- 

 ity. Distribution is accelerated by the move- 

 ments of the body, which set up haphazard 

 currents in the body fluid; but there is not 

 any very definite or sustained circulation of 

 the body fluid of Planaria. 



THE TUBULAR DIGESTIVE TRACT OF 

 THE EARTHWORM 



All animals above the evolutionary level 

 of the flatworms possess a tubular type of 

 enteron, generally similar to the digestive 

 tract of man. Such tubular systems allow for 

 a one-way passage of food materials from 

 the mouth to the anal opening. Food in- 

 gested through the mouth undergoes diges- 

 tion and absorption as it moves along the 

 length of the enteron, and finally the rem- 

 nants of the food are egested through the 

 anus. 



The earthworm and other Annelida (p. 

 647) possess a tubular enteron (Fig. 16-9) 



PARENCHYMA (MESODERM) 



which illustrates the advantages of such a 

 system. Annelida, like higher forms gener- 

 ally, have developed a sharply defined coelo- 

 mic cavity, which intervenes between the wall 

 of the digestive tract and the body wall (Fig. 

 16-10). Moreover, the gut wall has developed 

 a separate musculature, so that the digestive 

 movements of the animal are independent 

 of its external movements. The gut muscula- 

 ture takes care of churning, mixing, and 

 moving the food mass in the digestive tract, 

 while simultaneously the muscles of the body 

 wall are executing the various external re- 

 sponses of the animal. 



Tubular enterons are variously modified 

 in different higher animals. In the earth- 

 worm, the pharynx lies directly behind the 

 mouth and displays a relatively thick muscu- 

 lar wall (Fig. 16-9). The pharynx acts as a 

 sort of suction pump that expands and con- 

 tracts, sucking in small masses of moist 

 earth as the worm burrows into the soil. The 

 ingested earth, which contains considerable 

 rotting plant material, mixed with the hard 

 soil particles, then passes through the straight 

 cylindrical esophagus (segments 6 to 14) to a 



INTESTINE 



GIZZARD 



CROP CALCIFEROUS 



ESOPHAGUS GLANDS 



Fig. 16-9. Tubular digestive tract of the earthworm (diagrammatic). Note the differentiation of the suc- 

 cessive parts. 



