514 



COLLEGE ZOOLOGY 



absorbed. Certain foods such as water, oxy- 

 gen, mineral salts, and glucose and other 

 simple sugars, can be absorbed in the form 

 in which they are ingested; but most of the 

 food of animals consists of carbohydrates, 

 fats, and proteins, which will not usually 

 diffuse through membranes and protoplasm. 

 The proteins must be broken down into 

 amino acids, the fats into glycerol and fatty 

 acids, and the carbohydrates into simple 

 sugars. The products of digestion are used 

 by the body to: (1) produce energy when 

 they are oxidized, (2) be built up into proto- 

 plasm, (3) be stored for future needs, or 

 (4) manufacture special substances, such as 

 hormones. 



In most animals, certain mechanical proc- 

 esses take part in digestion such as the chew- 

 ing of food, the movement of food through 

 the digestive canal, and the grinding or mix- 

 ing activities within the digestive tract. 

 Chemical digestion is a process of h}drol- 

 ysis. During hydrolysis a complex com- 

 pound combines chemically with water, and 

 complex molecules are split into two or more 

 simpler molecules. For example, common 

 sugar (sucrose) when combined with water 

 gives rise to the two simple sugars, glucose 

 and fructose, as follows: 



Sucrose Water Glucose Fructose 



C12H22O11 + H2O -» CeHuiOe -f CeHoiOe 



The molecular formulas for glucose and 

 fructose are the same, but the atoms of 

 which they are composed are differently ar- 

 ranged; such substances are known as iso- 

 mers. Hydrolysis is brought about by en- 

 zymes which are secreted by the protoplasm. 

 Enzymes act only on the surface of food 

 particles, so, obviously, the previous me- 

 chanical breakup of food into small pieces 

 speeds up the digestive process. In higher 

 animals, glands are specialized for secreting 

 these enzymes. 



Two types of digestion can be recognized 

 in animals, intracellular and extracellular. 

 In protozoans and sponges, food particles 

 are digested inside food vacuoles within 



cells; and even in coelenterates such as the 

 hydra, and in certain higher animals, some 

 of the food is digested within cells. Hydras 

 also digest food outside of cells in a gastro- 

 vascular cavity. Coelenterates and flatworms 

 each possess a digestive cavity with only one 

 opening, whereas almost every animal above 

 them in the scale of life is provided with a 

 digestive cavity which has two openings, a 

 mouth and an anus. 



Digestive systems 



The digestive tracts of the earthworm, 

 crayfish, grasshopper, mussel, and vertebrates 

 are constructed on the same general plan. 

 The bodies of these animals are essentially 

 double tubes, as is clearly seen in the earth- 

 worm. The outer tube is the body wall, and 

 the inner tube is made up of the digestive 

 canal. Between the two tubes is a body 

 cavity. All organs which make up the diges- 

 tive canal, together with the other organs 

 that aid in digestion, constitute the digestive 

 system. 



Many of the peculiarities in digestive sys- 

 tems are correlated with the character of 

 the food and with the feeding habits of the 

 animals. For example, we have noted the 

 following: a planarian captures and ingests 

 food with the aid of a muscular, protrusible 

 phar}'nx; its digestive system includes three 

 main trunks and many lateral branches 

 which carry food to all parts of the body; a 

 circulatory system for this purpose is absent. 

 The earthworm possesses a muscular 

 pharynx for drawing food into the digestive 

 canal, a crop for storage purposes, a gizzard 

 for grinding up the food, and an intestinal 

 fold, the typhlosole, to increase the secreting 

 and absorptive surface of the intestinal 

 wall; lampreys have somewhat similar in- 

 testinal folds. The crayfish stomach is con- 

 stricted into two parts; the anterior part 

 grinds up the food and passes it through a 

 strainer into the posterior chamber. In the 

 grasshopper, food is stored in the crop and 

 ground up in the gizzard; 6 double gastric 



