In the study of cephalic structures the student should be quite critical. It is not uncommon for two workers examining the same species to find greater differences than one worker would find examining representatives of two families. Lateral and medial views are often quite helpful but without an en face view they may be meaningless. It is due to this fact that the majority of older descriptions of the cephalic strue tures of parasitic nemas must be considered valueless. In examining en face views great care should be taken in focus ing the microscope, and oil immersion is essential even in the study of the largest species.
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