124 THE ELEMENTARY NERVOUS SYSTEM 



muscular mechanism necessary for carrying out a food 

 response such as has been described (Parker, 1896). 



This autonomy of organs is nowhere better illustrated 

 than in the foot or pedal disc of the sea-anemones. Prob- 

 ably all actinians in which there is a well-differentiated 

 pedal disc have some powers of locomotion. These are 

 relatively slight and not often exercised in such a sea- 

 anemone as Metridium marginatum, which is almost ses- 

 sile; they are somewhat more evident in Sagartia lucice; 

 and they are decidedly characteristic of Condylactis. 

 Pedal locomotion in all these forms is accomplished by 

 a wave of contraction that arises on one side of the foot 

 and sweeps slowly across it to the opposite side. Appar- 

 ently this wave is not fixed in relation to the secondary 

 axes of the animal. It may arise at any point on the 

 periphery of the foot without reference to the internal 

 organization of the animal, but, having once arisen it 

 sweeps across the center of the foot so that a line drawn 

 from the point of origin through the center of the pedal 

 disc marks the direction of locomotion. In a specimen 

 of Condylactis whose foot measured 13 centimeters by 8 

 centimeters, the passage of a locomotor wave required on 

 the average 3 minutes and the animal progressed by means 

 of each wave on the average a little over 1 centimeter 

 (Parker, 1917 c). 



To what extent are the creeping movements of the 

 pedal disc dependent upon the animal as a whole? To 

 answer this question experiments were carried out on 

 Sagartia lucice. Fully expanded, attached specimens of 

 this actinian were suddenly cut transversely in two with 

 a pair of sharp scissors. The oral pieces thus cut off and 

 carrying with them in each case the whole of the oral disc, 

 tentacles and so forth, were discarded. The attached 

 pedal discs and remaining portions of the columns were 



