Feeding and Digestion 149 



Absorption. The fourth region is one of absorption. This region usually 

 overlaps the preceding one, and in many animals, such as the clams, starfishes, 

 and crustaceans, it is indistinguishable from the region of storage and internal 

 trituration. In the majority of species, including the sea urchins, sea cucum- 

 bers, insects, annelids, and vertebrates, the region of absorption is more or less 

 distinct. 



Conduction and Feces Formation. The fifth and final region is one of con- 

 duction and of formation of feces. This region is very conspicuous in such 

 animals as the terrestrial insects and vertebrates, in which an important func- 

 tion is the absorption of water from the fecal material, as in the insect hind- 

 gut and the vertebrate colon. The whole intestine of crustaceans and of most 

 molluscs comprises this region. 



Movement of Food Mass. Food material is propelled along the digestive 

 tract at a rate which permits effective digestion and absorption of the utilizable 

 constituents of the food. This propulsion is by one or both of two methods: 

 (1) ciliary activity, and (2) muscular activity. Ciliary activity appears to be 

 the exclusive means of movement through the digestive tract in such organisms 

 as the ectoprocts, entoprocts, and pelecypod molluscs. Muscular activity is 

 involved to a greater or less extent in food propulsion in other animals. Some- 

 times movement of somatic musculature associated with locomotion serves as 

 an effective force in the propulsion, supplementing the action of poorly 

 developed visceral musculature, for example in worms and holothurians. Well 

 developed visceral musculature operates effectively by itself in arthropods, 

 vertebrates, and probably cephalopods. In addition to propulsion the muscular 

 activity serves to triturate further and to mix the food with digestive fluids. 

 There may be peristaltic waves, standing waves, and pendular movement. The 

 action of emulsifying agents on fats is aided by the mechanical action of the 

 digestive organs. 



The coordination of the mechanical activity of the digestive tract has been 

 studied significantly only in the vertebrates, in which both endocrine and 

 nervous factors operate. Epinephrine inhibits gastrointestinal activity, whereas 

 acetylcholine increases intestinal motility, antagonized by atropine. Sym- 

 pathetic nerves increase and parasympathetic nerves decrease motility of the 

 gut (see Chapter 16). 



The time of retention of food in the digestive tract is inversely related to 

 the efficiency of the digestive processes. The rate of movement of food is 

 dependent on temperature in a fashion parallel to the effect of temperature 

 on the digestive reactions. A blood meal may be retained by a leech for several 

 months. "'^ One blood meal lasts the bug Rhodnius for one instar, whereas 

 food passes rapidly through caterpillars and grasshoppers. In fish several days 

 may be.required for digestion (three days in dogfish^*^*^). In warm-blooded 

 animals digestion is more rapid. About 70 per cent of an indigestible meal is 

 passed in feces in 24 hours in a rabbit, in 72 hours in man, in 18 to 20 hours in 

 a chicken. ^^ Food remains in the human stomach 3 to 4V2 hours, carbohy- 

 drates remaining a shorter time than proteins; in cattle, food remains in the 

 rumen an average of 61 hours, in the omasum 8 hours, and in the abomasum 

 3 hours. ^'^ A delicate balance exists between the kind of food, the rate of 

 digestion, and the rate of propulsion along the digestive tract; part of the 

 regulation may be local and much of it reflex in nature. 



