jgQ Comparative Animal Physiology 



not be available to higher vertebrates. ^- Cellulases are restricted to some 

 microorganisms and to a very few invertebrate animals. Pepsin and an acid 

 stomach are distinctly vertebrate characters which have been lost bv several 

 separate groups of bony fish, Dipnoi, some cypridonts, and others. ^^ Am- 

 phioxus and the cyclostomes are microphagous and have not acquired a pepsin. 

 In general, intracellular digestion is associated with the absence of tri- 

 turating mechanisms, and phagocytic cells, e.g. lamellibranch amoebocytes, 

 may possess a variety of enzymes. Extracellular digestion is more economical 

 of area of cell surfaces. 



The duration of retention of food in the digestive tract is inversely propor- 

 tional to the efficiency, in the sense of rapidity, of the digestive processes, and 

 the speed of extracellular digestion in warm-blooded animals is faster than that 

 at low temperatures. Also the amount of food eaten at a meal depends on stor- 

 age facilities: a herbivorous ruminant eats much more in bulk than a carni- 

 vore. A stomachless fish (roach, for example) can ingest less than 5 per cent of 

 its weight per meal, whereas a trout, which has a stomach, can take in 20 per 

 cent of its body weight at one feeding. ^^ The rate at which food passes 

 through the digestive tract is correlated with the kind of food and the effi- 

 ciency of the digestive enzymes. 



As shown in the preceding pages, the pH optimum for a given enzyme is 

 not always the same as the physiological pH at which it functions. Also the 

 pH optimum differs slightly according to substrate. However, in a general 

 way, the early phases of digestion are often in an acid medium and later phases 

 occur at a higher pH. 



The enzymes of an animal may change with development, as in many 

 butterflies, moths, blowflies, and bees, also as demonstrated in the decrease of 

 rennin in the mammaUan stomach. 



Animal hydrolytic enzymes are different from corresponding plant enzymes, 

 but given enzymes do not diff^er greatly from one animal group to another. 

 Animal amylases need to be activated by NaCl; plant amylases do not. ^''^ 

 Cathepsin functions extracellularly in invertebrates and intracellularly in 

 vertebrates. There is greater difference in the distribution of carbohydrases 

 than in that of other enzymes. Stereochemical or group specificity is marked, 

 and the existence of a given polysaccharidase is usually associated with the 

 presence of an enzyme for attacking corresponding sugars, as cellulase with 

 /8-glucosidase, amylase with a-glucosidase. 



Greatest success in the evolution of digestive function appears to be asso- 

 ciated with extracellular digestion, enzyme sequences, diversity of enzymes 

 and triturating mechanisms, together with an omnivorous diet. 



REFERENCES 



1. Abbott, R. L., /. Exper. Zool. 44:219-254 (1926). Digestive enzymes in Peri- 

 planeta. 



2. Allee, W. C, and Frank, P., Physiol. Zool. 21:381-390 (1948). Drinking by 

 fresh-water fish. 



3. Almy, L. H., ;. Am. Chem. Soc. 48:2136-2146 (1926). Proteolytic decomposi- 

 tion of herring. 



4. Autrum, H., and Graetz, E., Ztsc^r. vergL Phys'oi- 21:429-439(1934). Lipases 

 in leeches. 



