OF THE ALIMENTARY TRACT 5 



tozoon is non-cellular. To designate a protozoon as 

 a single cell or a unicellular animal seems inade- 

 quate or insufficient to some who are most familiar 

 with them, but there seems to be no way to avoid 

 the visual fact of a single celled body as long as the 

 cell is the generally recognized unit of life and until 

 we are in possession of deeper knowledge concerning 

 the fundamental factors or units of life beyond the 

 physical cell. Just as the fundamental unit of mat- 

 ter has changed in our conception with growing 

 knowledge, so will our ideas of the unit of living mat- 

 ter change and perhaps there will come a time when 

 cellular biology will be as inadequate as the molecular 

 idea may be in physics. 



Until then perhaps we should adhere to the con- 

 ventional and speak of Protozoa as unicellular ani- 

 mals, as differentiated from Metazoa, or higher 

 animals, in which the same or comparable vital organs 

 or systems are organized as separate recognizable 

 physical cells. It is common to think of the various 

 systems of the protozoa as of low order, primitive. 

 The size of their bodies, the accessibility of all parts 

 to food, water and oxygen absorption, the ease with 

 which waste materials may be cast out, the joining 

 of the principals of somatic and germ life in one 

 nucleus; all do away with the necessity for highly 

 complex alimentary, digestive, circulatory, respira- 

 tory, excretory, reproductive, and nervous systems. 

 Their organization is entirely adequate for their 

 lives, in fact the advantage which they have over 



