J 50 Comparative Animal Physiology 



SITE OF DIGESTION: INTRACELLULAR OR EXTRACELLULAR 



In evolution, digestion inside the cells which use the food products metab- 

 olically preceded extracellular digestion. In most animals there is both intra- 

 cellular and extracellular digestion, but certain animals have specialized in 



one or the other. 



Unicellular organisms show much variation in the site of action of digestive 

 enzymes. Autotrophic organisms (some bacteria, yeasts, and flagellates) absorb 

 only simple organic molecules, building them into complex ones. Some bac- 

 teria secrete digestive enzymes, such as proteases which liquefy gelatin, absorb 

 the products of intermediate size, and complete the digestion intracellularly. 

 Extracellular digestion by Protozoa occurs occasionally. The amoeboid cell 

 Vampyrella extrudes a cellulose-digesting enzyme onto a filament of Spirogyra, 

 digests a hole in the cell wall, then consumes the cell contents in 1 to 5 min- 

 utes. ^°^ It is probable that Protozoa which are intracellular parasites, such 

 as the malaria organism, destroy portions of cell membranes before pene- 

 trating the host cell. There are few convincing observations of enzymes freed 

 into a culture medium by Protozoa. Glaucoma is said to secrete an enzyme 

 which attacks peptone in the medium. ^^^ Most Protozoa take in food and 

 digest it in food vacuoles or in the cytoplasm. A food vacuole is extracellular 

 in the sense that the contents are separated from the cytoplasm by a membrane 

 which has very low permeability to many substances such as organic acids. 

 Enzymes are secreted into the food vacuole; moreover, enzymes contained 

 within live ingested food may act autolytically in the vacuole. Protozoa may 

 take in indigestible food and later pass the particles out; certain Protozoa may 

 show some selection of food. ^~^ 



Among metazoa, intracellular digestion appears to be correlated with slug- 

 gish habits, or with feeding on small particles. Extracellular digestion is asso- 

 ciated with the appearance of mechanisms for internal trituration and the 

 chemical breakdown of small particles or molecular aggregates. Extracellular 

 digestion appears to have evolved first as an adaptation for dealing with large 

 particles, to reduce them to such size that intracellular digestion could take 

 over; it is a sort of chemical trituration. The evolutionary trend has been 

 toward replacement of intracellular by extracellular digestion. Excellent ac- 

 counts of the site of digestion are given by Yonge. ^'"^ 



In sponges digestion is exclusively intracellular. Among triclad turbellarians 

 digestion is predominantly intracellular; the large cells lining the gut are 

 phagocytic and contain digestive vacuoles in which the breakdown of meat 

 can be followed.*^"' ^**^ In the rhabdocoel worms digestion occurs in the 

 copious lumen of the gut. In some of the Acoela, the cells lining the gut form 

 a syncytium in which digestion occurs; this is possibly an intermediate con- 

 dition between extracellular and intracellular digestion. "^ Intracellular 

 digestion is also predominant among coelenterates, although their gastrovascu- 

 lar cavity contains a protease which permits extracellular breakdown of protein 

 (Metridium,-^-^^ Hydra~~). All other digestion is carried on inside the 

 cells of the endoderm (Hydra^-), of the mesenteric filaments (Metridium, 

 ^^' ""), and of the lining of the manubrium and stomach of medusae.'^" 

 Similarly in madrepore corals the mesenteric filaments secrete proteases into 

 the coelcntcron, then absorb polypeptides and complete the digestion in vacu- 

 oles.^^** In carnivorous coelenterates some digestion probably also occurs by 



