DIGESTION 265 



Enzymes are produced which attack proteins, fats, starch and sugars. In 

 balanoglossids {Glossobalanus) the intestine and intestinal diverticula are 

 concerned with secretion and absorption. Studies of digestive ferments 

 have involved extracts of whole animals or gross regions: these digest 

 proteins, fats and carbohydrates. The digestive secretions of Amphioxus 

 are produced by the midgut and its diverticulum (liver). Extracts show 

 proteolytic, amylolytic and lipolytic activity. Digestion takes place initially 

 in the gut lumen and is completed intracellularly (9, 10, 44). 



Digestion in fishes, which is initiated in the stomach, is completed in the 

 intestine through the action of intestinal and pancreatic secretions. The 

 intestinal mucosa of selachians and teleosts secretes protease, amylase and 

 lipase. Opening into the anterior intestine of teleost fishes there is usually a 

 variable number of pyloric caeca, which secrete a complement of digestive 

 enzymes similar to those of the intestine. The pancreas, discharging into 

 the duodenum, secretes a powerful proteolytic enzyme, trypsin, and lipo- 

 lytic and amylolytic enzymes which supplement those of the intestine. 

 Digestion and absorption take place in the lumen of the intestine, and in 

 the pyloric caeca when present (4, 12, 13, 19, 37, 61, 63, 71). 



Digestive Enzymes 



From the scattered information available, dealing with many groups, it 

 is possible to recognize some correlation between the enzymatic comple- 

 ment produced and the nature of the diet. Omnivorous animals, such as 

 many polychaetes, echinoderms and decapod Crustacea, secrete proteolytic, 

 lipolytic and amylolytic enzymes and digest corresponding categories of 

 foodstuffs with equal facility. Secretions with strong proteolytic action 

 occur in carnivorous coelenterates, turbellaria, gastropods, cephalopods 

 and stomatopods, but other kinds of food materials are not necessarily 

 attacked with equal ease. Coelenterates, for example, appear to have little 

 or no ability to digest starch and sugars, and stomatopods lack an amylase 

 and invertase (sucrase). In herbivorous animals, on the other hand, ex- 

 tracellular proteoclastic secretions tend to be deficient or weak in action — ■ 

 for example in cirripedes, lamellibranchs, herbivorous prosobranchs and 

 tunicates. 



Types of Enzymes. During the course of digestion proteins are broken 

 down into their component amino-acids, carbohydrates hydrolysed into 

 monosaccharides, and fats hydrolysed into fatty acids and alcohol. 

 Enzymatic degradation of proteins and carbohydrates usually proceeds in 

 step-wise sequence, involving a number of enzymes, and we have noted 

 instances in which digestion may be initiated by secretions, later steps 

 being completed intracellularly. Special enzymes are present in certain 

 animals, e.g. cellulase, permitting exploitation of particular categories 

 of food, and ancillary substances may be produced in conjunction with the 

 working of special enzymes, e.g. bile salts. 



Lipases. Neutral fats are absorbed and hydrolysed intracellularly, or 

 they are digested in part in the gut lumen by secreted lipases, which have 



M.A. — 9* 



