168 REPRESENTATIVE SINGLE-CELLED ANIMALS 



are not in active locomotion by means of the flagellum. In some 

 euglenoids there are also movements that resemble peristaltic 

 waves as they traverse the cell body. All such activities are 

 termed " euglenoid," since they are commonly seen in the genus 

 Euglena (Fig. 91 B). Some of the larger euglenas regularly lose 

 their flagella and crawl about by a peculiar motion which is not 

 fully understood, although it has been ascribed to the existence of a 

 surface film of flowing protoplasm. This film is invisible but its 

 presence may be demonstrated by the movements of fine particles 

 adhering to its surface. The most characteristic movements, how- 

 ever, are those effected by means of the flagellum. 



The flagellum is a cyhndrical structure composed of an axial 

 filament surrounded by a sheath. This filament arises from the 

 hlepharoplast or basal granule, an enlargement within the cell, and 

 is the contractile portion (Fig. 91 A). The sheath apparent!} 

 functions as an elastic covering that tends to keep the flagellum in 

 an extended position. The action of such a flagellum as that of 

 euglena may be compared with the spiral waves which can be made 

 to pass along a rope that is tied at one end and held in the hand at 

 the other. The flagellum, however, is free at one end and makes its 

 own spiral waVes, which cause the cell to move through the water. 



Careful observation reveals that the euglena pursues a spiral 

 course (Fig. 91 D) which consists of three factors: (1) It moves 

 forward, progression; (2) it turns on its long axis, rotation; and 

 (3) it swings away from the axis of the spiral, swerving. One 

 can understand what happens by taking an object like a cane or a 

 larger cylinder and marking one side to indicate the shorter lip of 

 the euglena which is kept turned toward the axis of the spiral. If 

 the cylinder is then made to describe a cone and rotated to keep 

 this lip toward the axis of the cone, the factors (2) and (3) above 

 will be represented. By walking in the direction of the axis and 

 continuing to describe the cone, the factor (1) is added. In this 

 manner a euglena pursues a spiral path, always keeping the shorter 

 lip toward the axis of the spiral, and proceeding in a course, which, 

 if not a straight line, nevertheless enables it to steer what amounts 

 to a straight course " across the trackless deep." The spiral 

 movement of the organism is, of course, a result of the spiral beat- 

 ing of the flagellum. This mode of locomotion is the common one 

 in asymmetrical organisms, whether protozoa or other minute 

 forms that swim through the open water. By such means, an 



