Feeding and Digestion 151 



autolysis before the food is absorbed into the endodermal cells. In Tardigrade 

 and to a less extent in Rotifera and Brachiopoda, intracellular digestion pre- 

 dominates. ^''*° 



Molluscs show wide variety in the relative importance of extracellular and 

 intracellular digestion. '•'" Lamellibranchs digest protein, fat, and disaccha- 

 rides inside the cells of the digestive diverticula and in the wandering amoe- 

 bocytes which are abundant throughout the gut. ^^^ The amoebocytes 

 migrate into the wall and lumen of the gut in clams, charge their bodies with 

 food particles, digest them, and make their way again into the tissue spaces of 

 the animal. Starch is digested extracellularly by the action of amylase from the 

 crystalline style. Among gastropods the prosobranchs resemble lamellibranchs 

 in having only one extracellular enzyme, amylase, whereas carnivorous species 

 digest also protein and sugars extracellularly. ^''^ In snails such as Helix, 

 lacking amoebocytes in the gut, the hepatopancrcas or foregut diverticula 

 secrete several enzymes and the initial phases of digestion are extracellular; 

 the final phases occur inside the cells of the diverticula. Intracellular digestion 

 is prominent among nudibranchs. ^^^ 



Concurrent intracellular and extracellular digestion also occurs in echino- 

 derms which secrete enzymes upon food while it is still outside the body. In 

 addition, amoebocytes are abundant in the lumen of the echinoderm digestive 

 system, and cells of the pyloric and intestinal caeca are phagocytic. The arach- 

 noid Liniuliis carries out preliminary digestion of its prey extracellularly but 

 completes digestion in the primitive way in the cells of the digestive diverticula. 



Digestion is predominantly extracellular among the Annelida, Crustacea, 

 Insecta, Cephalopoda, and Chordata.^''" Digestion is extracellular in all 

 classes of vertebrates, but many organs such as liver, spleen, and kidney con- 

 tain intracellular enzymes which are capable of hydrolizing foods. Vertebrate 

 cathepsins are intracellular proteolytic enzymes. Inside the cells these enzymes 

 may function in protein synthesis. It is possible that terminal digestion, even 

 in mammals, may be intracellular. From observations on intestinal loops in 

 dogs the secreted enzymes are evidently insufficient to account for the break- 

 down of peptone, sucrose, and lactose; hence it is possible that these substances 

 may be digested inside the mucosal cells where peptidases and sucrase are 

 found. ^^' ^^^ Similarly in the silkworm, Bomhyx, starch digestion is extra- 

 cellular, and subsequent sugar breakdown takes place inside the gut cells. ^^* 



The animal groups with extracellular digestion are the most active and by 

 many standards the most successful of animals. They have mechanisms for 

 pulverizing their food and can consume, digest, and eliminate residues more 

 rapidly than do animals with intracellular digestion. Further, the surface 

 required for extracellular digestion is much less extensive, compared with the 

 ramified chambers of digestive diverticula in molluscs and flatworms. Although 

 extracellular digestion has evolved several times, it has not always developed 

 simultaneously for different foods in the same animal. 



HYDROGEN ION CONCENTRATION OF DIGESTIVE FLUIDS 



There are two general patterns of secretion and locus of action of the extra- 

 cellular digestive enzymes. Among most invertebrate animals with extra- 

 cellular digestion the various enzymes are secreted together into one chamber 

 where most digestion occurs. A sequence of chambers into which specific 



