462 Physiology 



ment of food along this canal. Such a canal resembles to some extent the 

 long sausage-shaped food vacuoles formed by Paramecium in certain salt 

 solutions (94), but other workers have failed to find a tubular digestive 

 system in ciliates (103, 182). 



After formation of the food vacuole, the contents apparently become 

 acid sooner or later. As reported in Pararnecium cau datum (103, 522) and 

 Actinosphaerium eichorni (212), a drop to pH 4.0-4.3 occurs after a time. 

 In Amoeba, the pH of the vacuolar fluid falls to some point between 4.0 

 and 6.5 (389). The acidity of the vacuole in P. catidatum is said to ap- 

 proach that of 0.8N hydrochloric acid; in certain other ciliates, less than 

 that of 1 X 10-^ N acid (413). The origin of this acid is uncertain, al- 

 though Mast (389) attributed it to the respiration of ingested organisms 

 and their later autolytic changes. During later stages of digestion and 

 absorption, there is a gradual rise in pH, sometimes to about pH 7.0 

 in old vacuoles containing undigested residues. 



Undigested materials are usually eliminated through a definite area 

 (cytopyge) in Protozoa with a well-developed cortex. In various Peritri- 

 chida the contents of the old vacuole are discharged into the vestibule 

 (Chapter VII). In many other ciliates, the cytopyge lies at the surface 

 somewhere in the posterior half of the body. A similar differentiation also 

 may be found in such flagellates as Peranema trichophorinn, in which the 

 supposed cytopyge is a small area in the postero-lateral body wall lacking 

 the usual cortical inclusions. 



Digestion of proteins^^ 



Protozoa which ingest solid food presumably are equipped with 

 digestive enzymes, and would thus be expected to produce both endo- 

 peptidases (proteinases) and exopeptidases (peptidases). Didinium na- 

 siitum seems to be an interesting exception which depends upon its 

 ingested prey for a supply of peptidases (101). "Glaucoma" pyriformis pro- 

 duces an endopeptidase active in the pH range, 2.2-9.6, with an optimum 

 at about pH 6.0 (311, 312); comparable enzymes of Plasmodium gal- 

 linaceum are more active at pH 6.5 than at 7.4 (408). Exopeptidases have 

 been reported in Parameciu^n caudatum, Frontonia sp., and Amoeba 

 proteus (204). Certain trypanosomes likewise produce exopeptidases and 

 also an endopeptidase of the kathepsin type, but no enzymes of the pepsin 

 or trypsin types (302). 



The ability to digest proteins also has been reported for Euglena 

 gracilis (173, 237, 377) and for such a saprozoic flagellate as Leishmania 

 tropica (498). Digestion in such cases is presumably extracellular but it 

 is uncertain whether the enzymes are eliminated during life or are re- 

 leased by disintegration of the flagellates. 



^* A brief general discussion of digestive enzymes will be found in Baldwin's (10) 

 monograph. 



