250 



GENERAL PHYSIOLOGY 



size, the unusual simplicity of their arrangement in rows, 

 and the fact of the rhythm of their beat being frequently very 

 slow, these swimming-plates serve as no other object does for 

 experimentation and observation. As was said above, the plates 

 are formed of many cilia cemented together, but each cilium 

 evidently makes exactly the same movement as the whole plate, 

 so that observations made upon the whole plate may be transferred 

 to the conditions in a single cilium. On account of the size of 

 the object observations can be made with the naked eye or with a 

 weak lens. If a single swimming-plate be observed in profile, 

 it is seen that in the resting-position it lies flat against the body, 

 so that it shows two curves, a greater one of smaller radius 

 immediately above the base, and a smaller one of greater radius 

 and in the opposite direction in the upper half (Fig. 110). This 



FIG. 110. Swimming-plates of Beroe in profile, a, In the resting-position; b, in the position of 



extreme contraction. 



is the position of rest. If now the plate performs a stroke, the 

 lower curve beginning from the base of the cilium extends itself 

 completely, even giving place to a slight curve in the opposite 

 direction. Hence, in the position of extreme swing the plate 

 stands erect with a slight curve toward the opposite side. The 

 progressive phase of the stroke is thereby completed. Now 

 follows the retrogressive phase, in which the plate falls back again 

 into its position of rest, the original curve at the base gradually 

 coming back until the plate again lies against the body. The 

 retrogressive phase proceeds more slowly than the progressive. 

 Because of this fact and by means of the upper curve into the 

 special significance of which we shall go no further it is rendered 

 possible that the motor effect of the progressive phase is not balanced 

 by the retrogressive phase; otherwise the animal would remain 

 continually in the same place in the* water. The movement of 



