DIGESTION 269 



it is known to occur in terrestrial pulmonates and arthropods (32, 41, 42, 

 45, 51, 58, 73). 



Conditions affecting Digestion 



Hydrogen Ion Concentration. Intracellular digestion in general involves 

 two main phases, a preliminary acid phase when the food organism is 

 killed and some digestion takes place, and a succeeding alkaline phase 

 during which digestion of the food is carried to completion. The activity 

 of extracellular digestive enzymes is affected by the hydrogen ion concen- 

 tration of the medium, and such enzymes show a narrow or broad pH 

 range of optimal activity. Much attention has been given to determining 

 both the hydrogen ion concentration of gut regions where the digestive 

 enzymes act and the pH optima of different categories of enzymes in lower 

 animals, and pertinent tables may be found in reviews by Kruger (40) 

 and Prosser (57). 



The acidity of the alimentary tract depends on the kind of food eaten, 

 and on whether the animal is fasting or has eaten; it varies during the 

 course of digestion, as the result of secretion and resultant breakdown of 

 foodstuffs; and it tends to change in some regular manner in successive 

 regions of the gut. Sometimes the acidity actually found in a given region 

 of the gut cavity (the physiological pH) approximates the optimal pH 

 for the enzyme or enzymes secreted into it, but this is not invariably so. 



In coelenterates (Scyphozoa, Anthozoa), the pH in the coelenteron falls 

 during digestion, when secretion is taking place and proteins being broken 

 down. Thus, the pH in the coral Fungia is about 7-8 when fasting, and drops 

 to 7-0 after feeding. This latter value is optimal for the extracellular pro- 

 teinase of Fungia. When digestion is completed and the coelenteron evacu- 

 ated, the pH rises once more. In starfishes (Patiria, Asterias) the acidity in 

 the pyloric caeca appears to be regulated at about pH 6-7; this level is 

 optimal both for ciliary viability and proteolytic action. An acid secretion 

 from the lips and pharynx of Echinus lowers the pH to 5-9, whereas the 

 acidity in succeeding gut regions gradually rises to pH 6-8 (62). 



Molluscs, we have already seen, present much variety in digestive 

 mechanisms. In those lamellibranchs and gastropods possessing a crystal- 

 line style, the entire gut shows an acid reaction and the style, moreover, 

 modifies the pH of the gut. Average pH values for the oyster (Ostrea 

 edulis) diXt: mantle cavity, 6-9; oesophagus, 5-8; stomach, 5-5; style, 5-2; 

 digestive diverticula, 5-8 ; midgut, 5-8 ; rectum, 5-9. The lowpH of the stomach 

 results from continuous dissolution of the head of the crystalline style, 

 which rotates against the gastric shield, and the acid reaction so produced 

 is about optimal for the amylase which is also released by the style (Fig. 

 6.9). In the herbivorous tectibranch Aplysia punctata the pH drops from 

 6-2 in the buccal cavity to 4-9 in the gizzard, and rises to 7-6 in the posterior 

 intestine; in the octopus the pH also falls from about 6-8 in the gullet to 

 5-5-5-8 in the stomach (35, 59). 



It is not uncommon among invertebrates with a well-differentiated gut 



