164 



Comparative Animal Physiology 



digestion is indicated by pH optima in other animals as well (e.g. lamelli- 

 branchs).^-' ^^^ Trypsin may be inhibited by agents which activate cathep- 

 sin. Cathepsin has not been reported for any insect except from the crop and 

 mid-gut of dragonfly nymphs (Table 26). 



pH 



Fig. 30. Digestion of gelatin by pyloric caeca of starfish at different hydrogen ion 

 concentrations, with and without cyanide. From Sawano."' 



0.20 



O.I 5 



005 



Fig. 3 1 . Digestion of gelatin by extract of liver of cephalopod, Polypus, at different 

 hydrogen ion concentrations, with and without cyanide. From Sawano"" 



It is probable that several trypsins exist. Some, such as those of several 

 insects' '"'" and Limulus, '^•* are activated by mammalian enterokinase. 

 Others, such as the trypsin of Paramecium,^'^^ Mwrex,"'* and Maja,^'^^ 

 are not activated by enterokinase. The few attempts to find activating agents 



