Feeding and Digestion I55 



of a blowHy larva feeding on meat is alkaline. The mid-gut and stomach of 

 many animals is a region of acid secretion (pH 5-6), although in no group of 

 invertebrates is the mid-gut as acid as it is in mammals. Farther back in the 

 digestive tube the pH rises, sometimes to alkaline levels (Table 25). In many 

 molluscs the entire gut is acid, the stomach most so, and the gastric acidity is 

 controlled in part by the crystalline style. The mid-gut fluid is alkaline in 

 Trichoptera, Lepidoptera, and some Coleoptera, "'' although the mid-region 

 of the mid-gut of blowfly larvae may have a pH of 3.2. Digestive fluids of the 

 silkworm are as alkaline as pH 9.5. '•■^-» In corals Hving in sea water of pH 8.2 

 the coelenteric fluid had a pH in starvation of 7.78; two hours after feeding 

 the pH was 7.05. ^^^ 



In free-living Protozoa the pH of food vacuoles can be followed if food or 

 fluid medium is stained with appropriate indicators. Greenwood in 1894 

 pointed out that in Carchesium the food vacuole is first acid and then alkaline, 

 and that most digestion occurs in the alkaline phase. This sequence holds for 

 numerous ciliates; -^^ Paramecium reaches the highest acidity. ^^^ Mast 

 followed individual stained food vacuoles in Amoeba proteus, "^ several 

 peritrichs, "'■* and Paramecium caudatum, "^ and in each the food vacuole 

 becomes acid soon after it is formed; while it is acid its volume decreases. 

 After a few minutes a food vacuole becomes more alkaline and enlarges rapidly. 

 It is postulated that products of hydrolysis during the acid phase increase the 

 osmotic concentration in the vacuole; then alkaline fluid enters from the 

 cytoplasm. The source of the acid is not entirely certain, but there is good 

 evidence that the vacuolar membrane does not permit loss of acid during the 

 initial period. Some of the acid— perhaps all of it in Amoeba, where the lowest 

 pH is 3.5 to 4.0— comes from food organisms contained in the vacuole, and 

 death of prey occurs in the acid phase. In the ciliates— such as Paramecium, 

 where the pH may go as low as 1.4— some acid is probably secreted by the 

 pharynx. As the vacuole swells in the alkaline phase the pH may rise above 

 that of the cytoplasm; enzymatic digestion then proceeds. 



The digestive secretions of most animals are acid in the early and alkaline 

 in the late phases of digestion. This general pattern holds whether the enzymes 

 are secreted together into one chamber or follow in sequence. Variations may 

 be correlated with food habits and enzyme complement of different animals. 



STIMULATION OF SECRETION OF DIGESTIVE FLUIDS 



When a digestive organ does not contain food, there is usually a low-level 

 continuous secretion of fluid which may have a lubricating function, and 

 which diff^ers in composition from the secretion during active digestion. Also 

 in animals which feed most of the time, as in pelecypods by continuous ciliary 

 propulsion of food or as in continually browsing scavengers, the flow of 

 digestive fluid must be continuous. However, in most animals feeding is 

 periodic, and secretion is elicited to correspond to the presence of food. Gland 

 cells can be stimulated by chemical agents in tissue fluids which bathe them, 

 by direct mechanical stimulation, by nerve impulses, or by combination of all 

 three methods of stimulation. 



The control of digestive secretion has been examined very extensively in 

 mammals, particularly in the dog, and hardly at all in lower animals, and the 

 mammalian mechanisms are well described in medical physiology texts. In 



