296 - Multicellular Animals, Especially Man 



ENTERON 



COELOM 



HYDRA 



PLANARIA 



EARTHWORM 



Fig. 16-10. Diagrams of cross sections of Hydra, Planaria, and earthworm. 



thin-walled distensible chamber, the crop. 

 While passing through the esophagus the 

 food mass is mixed with the juices from three 

 pairs of multicellular glands, the calciferous 

 glands (Fig. 16-9). These secretions are rich 

 in calcium carbonate, which may influence 

 the digestive reactions. When the crop ac- 

 cumulates sufficient food, it passes the mass 

 on to the thick-walled gizzard (segments 17 

 and 18), which is a grinding organ. The 

 grinding action of the gizzard results from 

 the strong rhythmic contractions of its thick 

 muscular walls. Bits of leaves and other small 

 masses of organic material are cut into 

 smaller and smaller pieces, as they are re- 

 peatedly compressed between the sharp par- 

 ticles of the soil. This grinding action is im- 

 portant because it increases the exposed 

 surface of the food mass and permits the 

 digestive enzymes to act more effectively. 



The intestine of the earthworm is the last 

 and longest part of the digestive tract (Fig. 

 16-9). This thin-walled tube extends straight 

 back from the gizzard to the anal opening, 

 at the posterior extremity of the worm. While 

 the food mass moves slowly along the intes- 

 tine, digestion and absorption are completed. 

 Many unicellular digestive glands are pres- 

 ent in the epithelial lining of the intestine, 

 and these glands secrete carbohydrases, pro- 

 teases, and lipases upon the passing food. 

 The sugars, amino acids, glycerol, and other 

 relatively simple end products derived from 

 the hydrolysis of the organic foods dissolve 

 in the water fraction of the moist food mass; 

 and as these end products accumulate, they 



tend to be absorbed by diffusion, through 

 the endoderm into the blood, which flows 

 continuously through the capillaries in the 

 wall of the intestine. 



THE DIGESTIVE SYSTEM OF MAN 



The human digestive tract, like that of 

 Vertebrata generally, is a tubular system, 

 possessing a characteristic number and ar- 

 rangement of the several parts (Fig. 16-11). 

 Food is ingested through the mouth into the 

 oral cavity, where it is chewed and mixed 

 with saliva (Fig. 16-11). Then the food is 

 swallowed and passes quickly through the 

 short pharynx and the long esophagus to the 

 stomach. The stomach is a thick-walled, 

 muscular, saclike portion of the enteron, 

 which churns the food mass sometimes for as 

 long as 3 to 5 hours, while the food is acted 

 upon by the gastric juice. This juice pours 

 into the stomach cavity from thousands ol 

 microscopic glands, the gastric glands, pres- 

 ent in the lining of the stomach wall (p. 299). 

 When the food has been reduced to fluid 

 form, it is passed on into the small intestine 

 (Fig. 16-11). This long (23 ft) and relatively 

 narrow part of the digestive tract is highly 

 coiled, and the many coils of the small intes- 

 tine are all crowded together in the abdom- 

 inal cavity. The small intestine receives the 

 food mixture from the stomach and, in the 

 course of about 10 hours, transmits the rem- 

 nants of the food to the large intestine (Fig. 

 16-11). Meanwhile, pancreatic juice from the 

 pancreas and bile from the liver (Fig. 16-11) 



