DIGESTION 273 



dissolution of the crystalline style. There is always enzyme present, there- 

 fore, to deal with the constant food intake. Fabrication of the style, how- 

 ever, ceases under adverse conditions. 



In other animals we find that secretion is elicited by the presence of food. 

 The principal extracellular enzyme of coelenterates is a proteinase. There 

 is little digestive action in a fasting animal, while the presence of animal 

 material (meat) stimulates secretion. In several groups (gastropods, inter- 

 tidal lamellibranchs, decapod Crustacea) digestive secretion is a rhythmical 

 process, which is accelerated by the presence of food. Secretion is holo- 

 crine (Potamobius) or merocrine (Atya). In the former condition the 

 individual cells are sacrificed, and periodic replacement takes place by cell 

 division at the ends of the diverticula. Merocrine secretion involves dis- 

 charge of cellular granules (39, 50, 67), 



Secretion in cephalopods is markedly different from that in other 

 molluscs and is related to the active predatory habits of these animals. 

 In the squid, pancreatic secretion accumulates in the caecum between 

 meals and passes into the stomach during gastric digestion. When this 

 occurs, the caecal valve is held open and contractions of the caecal sac 

 drive out the stored fluids. The hepatic secretion, on the contrary, is 

 liberated only during digestion, and control is effected by the hepatic 

 sphincter (16). 



The factors which regulate discontinuous secretion — which evoke it 

 prior to, and during the course of, a meal — have rarely been subjected to 

 any precise analysis in invertebrates, and the situation in fishes is not less 

 obscure. There is a continuous secretion of small amounts of gastric juice 

 in fasting selachians. But in contradistinction to the situation in tetrapods, 

 gastric secretion is not subject to nervous control. It is true that stimulation 

 of the sympathetic nervous system and injection of adrenaline in rays may 

 produce inhibition of normal secretion, but this can be ascribed to vaso- 

 constriction and the attendant drop in blood flow. Even less is known about 

 the situation in teleosts, where gastric secretion is certainly discontinuous and 

 depends on the presence of food. The secretion of pepsin under the influence 

 of pilocarpine has, however, been detected in intact plaice (Pleuronectes). 



In fasting fish the intestinal fluids contain little trypsin, but this increases 

 greatly when food enters the duodenum. Secretin (a hormone provoking 

 pancreatic secretion) has been detected in the fish gut, and it is found that 

 injection of secretin or the introduction of HC1 into the duodenum of the 

 ray causes the pancreas to secrete. Consequently, there appears to be a 

 hormonal regulation of pancreatic secretion in fish, mediated by secretin 

 and comparable to that found in mammals. Passage of acid chyme into 

 the stomach excites this mechanism and results in pancreatic secretion 

 into the duodenum (3, 5, 11, 12, 14, 54). 



TRANSPORT OF FOOD THROUGH THE GUT 



Ciliary action and muscular contractions propel the foodstuffs and fluids 

 along the alimentary canal while they are being comminuted, digested 



