PEDAL LOCOMOTION IN ACTINIANS 115 



required 2.5 minutes to traverse the disc, the most rapid 1 min- 

 ute, the average time being 1.65 minutes. The distance pro- 

 gressed as a result of each wave was on the average 1.2 mm. 

 Each wave began by a general contraction of the posterior edge 

 of the pedal disc whereby that portion of the disc was Ufted off 

 the substratum and crowded toward the front. This wave of con- 

 traction gradually proceeded toward the middle of the disc and 

 the posterior edge was then brought down on the substratum a 

 little in advance of its former position. The wave now reached 

 the anterior edge of the disc and, while the middle was being re- 

 attached to the substratum, the anterior edge, now freed, was 

 advanced and finally brought down somewhat in front of its 

 former position. Thus the passage of a single wave carried the 

 whole pedal disc forward for a short distance and as a result the 

 animal changed its location. It is doubtless the front portion 

 of this wave that Torrey ('04, p. 204) speaks of in Sagartia da- 

 visi as a multicellular ameboid process by which this species could 

 creep more than an inch an hour. 



In Actinia bermudensis the pedal waves were quite hke those 

 seen in Sagartia except that they were of more considerable di- 

 mensions. In a specimen whose pedal disc measured 30 by 27 

 mm. the complete passage of a single w^ave required from 3 to 3.25 

 minutes and at each wave the animal progressed from 4 to 4.5 

 mm. In Metridium, whose pedal disc is about the same size as 

 that of Actinia bermudensis, the locomotor wave passes over 

 the disc, according to McClendon ('06), in the short interval of 

 about a minute. 



In Condylactis, because of its large size, the waves were more 

 satisfactorily studied than in either Sagartia or Actinia. In a 

 specimen whose pedal disc measured 130 by 80 mm., the waves 

 passed over the disc in from 3 to 4 minutes and for each wave 

 the animal progressed a distance varying from 5 mm. to as much 

 as 17 mm. In another instance 10 waves were seen to pass in 

 30 minutes and the animal progressed in this interval 114 mm. 

 Thus the average period of each wave was 3 minutes and the 

 average distance advanced for each wave was 11.4 mm. As the 

 wave passed over the pedal disc of Condylactis, it was easy to 



