168 THE ELEMENTAEY NERVOUS SYSTEM 



pedal disc and adjacent parts. It is, however, an activ- 

 ity of the disc as a whole. Locomotion has never been 

 observed in pieces of the pedal disc. When actinians are 

 cut in such a way that the fragments retain only parts 

 of the original pedal disc, they remain attached to the 

 substratum by their pedal surface, but they never ex- 

 hibit locomotion. It is only after regeneration has set in 

 and a new pedal disc has been established that loco- 

 motion recommences. Creeping then is a response which 

 calls for a much more unified mechanism than feeding, 

 and Lukas (1905) is probably correct in regarding it as 

 a response that gives evidence of the highest form of 

 nervous activity thus far discovered in actinians. That 

 it is indicative of a primitive form of desire and hence 

 discloses the beginnings of consciousness as asserted by 

 Lukas (1905) can be neither proved nor disproved. But 

 of its importance as showing a certain amount of unity 

 in actinians there can be not the least doubt. 



Another line of investigation that is suggestive of 

 more than the simplest form of nervous activity in actin- 

 ians is the modifiability of their responses. This sub- 

 ject has been investigated by Jennings (1905), who has 

 shown its significance by direct experiment. If a drop 

 of water is allowed to fall on the surface of the water in 

 which an expanded Aiptasia rests, the animal will usually 

 retract. After expansion a second drop often fails to 

 call forth any such response and in fact it is necessary to 

 allow, as a rule, an interval of five minutes before a sec- 

 ond response can be elicited. Thus the earlier stimulus 

 influences the neuromuscular apparatus of the sea-anem- 

 one in such a way that a repetition of the stimulus is 

 not followed by a response. To put the matter as Jen- 

 nings does, tlio previous history of an organism has its 



