PRINCIPLES Of PHYSIOLOGY Q\ 



The body of the coelenterate consists of two layers of cells; the 

 inner one is specialized for digestion and absorption. Food— small ani- 

 mals and plants caught by the tentacles— passes through the mouth and 

 enters the central gastrovascular cavity. The endoderm cells secrete 

 digestive enzymes into this cavity and some digestion occurs. This is 

 extracellular digestion, occurring in a special digestive cavity, and is 

 found in most animals. Some partly digested food particles are taken up 

 by the endoderm cells in food vacuoles in which intracellular digestion 

 occurs. There is no separate anal aperture; undigested wastes leave the 

 gastrovascular cavity by the mouth. Digestion in the fiatworms, such as 

 planaria, is similar to that in the coelenterates: food enters and wastes 

 leave the branched digestive tract via the same opening and digestion 

 is partly extracellular and partly intracellular. The gastrovascular cavity 

 of the flatworm is greatly branched and the branches extend throughout 

 most of the body, thus facilitating the distribution of digested food. 



In most of the rest of the invertebrates, and in all the vertebrates, 

 the digestive tract is a tube with two apertures; food enters by the mouth 

 and any undigested residue leaves by the anus. The digestive tract may 

 be short or long, straight or coiled, and subdivided into specialized 

 organs. These organs, even though they may have similar names in 

 different kinds of animals, may be quite different, and may even have 

 different functions. 1 he digestive system of the earthworm, for example, 

 includes a mouth, a muscular pharynx which secretes a mucous material 

 to lubricate the food particles, an esophagus, a soft-walled crop where 

 food is stored, a thick muscular gizzard where food is ground against 

 small stones, and a long straight intestine in which extracellular diges- 

 tion occurs and through the wall of which the food is absorbed. Many 

 invertebrates— worms, squid, crustacea, sea urchins— have hard, toothed 

 mouthparts for tearing off and chewing bits of food. 



The details of the vertebrate digestive system will be given in Chap- 

 ter 26. It is similar in basic plan to that of the earthworm, but has 

 undergone further evolution and specialization. There is a separate 

 small intestine, Avhere most digestion and absorption occurs, and a fol- 

 lowing large intestine in which digestion and absorption, especially the 

 absorption of water, are completed. The vertebrate digestive system also 

 includes the liver and pancreas, connected to the small intestine by 

 ducts. These large digestive glands produce, among other things, certain 

 enzymes and other substances required for digestion. 



The Digestive Process. Digestion, whether in ameba or man, in- 

 volves the splitting of complex molecules into simpler ones by the addi- 

 tion of water, a process called hydrolysis. There are specific hydrolases 

 for the enzymatic splitting of proteins, fats and carbohydrates. The 

 digestive enzymes of vertebrates include the protein hydrolases pepsin, 

 secreted by the stomach, trypsin and chymotrypsin, secreted by the pan- 

 creas, and several peptidases secreted by the pancreas and intestinal 

 mucosa. Lipases, which split fats, are secreted by the pancreas. The 

 carbohydrate-splitting enzymes include ptyalin, secreted by the salivary 

 glands, amylase, secreted by the pancreas, and maltase, sucrose and 

 lactase secreted by the intestinal mucosa. Each enzyme has a specific pH 



