PEDAL LOCOMOTION IN ACTINIANS 117 



advance of its former position. At D the wave has reached the 

 anterior edge, which is now beginning to be moved forward. At 

 E the whole disc is at rest again but in a position in advance of 

 its former situation. 



I think there can be no doubt that this type of movement 

 was observed by Gosse ('60, p. 81) in Sagartia paUida though 

 somewhat inaccurately described by him. Gosse states that this 

 species, which crept away from the hght and covered as much 

 as thrice its length in a quarter of an hour, accomplished this 

 movement by pushing forward the front part of its pedal disc 

 and attaching this portion, after which the hind part was loos- 

 ened, drawn up to the front part, and reattached. This descrip- 

 tion, which agrees with that recently given by Fleure and Walton 

 ('07, p. 218), imphes a type of motion like that of a measuring- 

 worm, and is probably a slightly inaccurate account of the wave 

 movement seen by McClendon and me, from which it differs after 

 all only slightly. 



In all the creeping actinians that have come under my obser- 

 vation I have never noticed more than a single wave on the pedal 

 disc at one time. In my experience a given wave runs its full 

 course and completely disappears at the front edge of the disc 

 before a second wave begins to form at the hind edge. McClen- 

 don ('06), however, states that he has seen two waves on the 

 disc of Metridium at the same time. I know of no reason why 

 this may not occur especially when the direction of creeping is 

 well established. 



The direction in which the pedal wave is propagated always 

 agrees with that of the locomotion. The waves on the actinian 

 disc may, therefore, be called direct waves to borrow a term 

 introduced by Vies ('07) for the waves on the foot of the gastro- 

 pod. But this is not the only resemblance of the actinian wave 

 to that on the gastropod foot. As the preceding description 

 shows, the two movements are mechanically identical, for in the 

 locomotion of the gastropod, as in that of the actinian, each point 

 on the foot is successively lifted, moved forward, and put down 

 (compare Parker, '11). 



In another respect also the creeping of the actinian agrees 

 with that of the gastropod. As Osburn ('14, p. 1165) pointed out 



