122 G. H. PARKER 



pedal disc is like an actinian tentacle which on being cut off will, 

 if properly handled, carry out temporarily all the reflexes that 

 it exhibited while it was a part of the animal as a whole. 



The creeping of an actinian is by no means a simple process. 

 Not only is a rather complex wave of locomotion made to pass 

 over the pedal disc from one side to the other, but this wave 

 may originate at any point on the edge of the disc and pass 

 through its center to the opposite edge; in other words, the disc 

 appears to possess potentially an infinite number of axes of loco- 

 motion. From this standpoint its strictly radial structure con- 

 forms well with its activities, for a system of circular and of 

 radial fibers in a horizontal plane combined with a set of ap- 

 proximately vertical fibers gives just that structural combina- 

 tion necessary for unrestricted radial locomotion. The effective 

 symmetry of the actinian pedal disc is, then, a locomotor sym- 

 metry for it is strictly radial; the symmetry of the oral disc, as 

 already pointed out, is bilateral and is in no direct way connected 

 with locomotion. 



What it is that induces the formation of locomotor waves on 

 any particular side of the pedal disc of an actinian, is not easy 

 to discover. Probably many stimuli are effective in this re- 

 spect. The fact that Sagartia and many other actinians creep 

 away from light suggests that the light itself may stimulate the 

 mechanism concerned with the formation of the locomotor 

 waves, for in negative animals such as these the light falls most 

 strongly on that face of the animal where the waves originate. 



Sagartia likewise commonly creeps up the sides of glass jars 

 and it will do this even when the top of the jar is closed and 

 contains no air. Such a reaction would, therefore, seem to be of 

 a geotropic kind and dependent perhaps upon the deforming pres- 

 sure exerted on the lower edge of the pedal disc of this actinian 

 when attached to a vertical support, for the body of Sagartia is 

 slightly heavier than sea water and would tend to sag downward 

 from a horizontal position. However this may be, some stimu- 

 lus must be present to start from a given side the locomotor 

 wave which then courses in a determined direction across the 

 pedal disc to be followed often by others whose presence calls 



