258 H. P. KJERSCHOW AGERSBORG. 



Say) locomotion is accomplished without pedal waves; (2) with 

 pedal waves, or rhythmic locomotion (Dolabrifcra virens Verrill, 

 Tectarins nodulosus Gmel., Nerita tcsscllata Gmel., Chiton tubercu- 

 latus Linnaeus, etc.). "In rhythmic locomotion the waves may 

 run from the posterior to the anterior, that is, direct; or the re- 

 verse, that is, retrograde. The foot may exhibit one, monotaxic; 

 two, ditaxic ; or four, tctrataxic, series of waves. In the ditaxic 

 foot the waves may be alternate or opposite. . . . Locomotion is 

 the cumulative result of local forward motion on the part of one 

 section of the foot after another till the whole foot has been 

 moved." The essential act of pedal locomotion of all creeping 

 gastropods may be exhibited by the retrograde wave movement on 

 the foot of Chiton tuberculatus, in which, according to Parker 

 ('14), the locomotor waves run from anterior to posterior. In 

 this type, " The waves extend the whole width of the foot and are 

 from five to six mm. in antero-posterior extent. They represent 

 an area of the foot temporarily lifted from the substrate. . . . 

 Excepting in the region of the wave, the foot is firmly attached to 

 the substrate; hence at any moment from nine tenths to four fifths 

 of the foot is fixed and the remainder free." Pedal locomotion of 

 Actinians (Mctridium marglnatum Milne-Edw., Sagartia lucice 

 Verrill, Condylactis passiflora Duch. and Mich., and Actinia bcr- 

 mudcnsis Verrill) is interestingly accomplished by a wave-like 

 movement which progresses over the pedal disc in the direction of 

 locomotion (Parker, '17). "In the actinian locomotor wave each 

 point on the pedal disc is successively raised from the substratum, 

 moved forward, and put down." Writing on the locomotion of 

 the sea-hare, Aplysta calif ornica Cooper, this same author ('i/a) 

 says : " Pedal locomotion in Aplysia is due to monotaxic retrograde 

 waves which lift the foot locally and temporarily from the sub- 

 strate, making it thus to move forward with freedom, while the 

 rest of the foot for the time being holds the snail in place by many 

 small areas of local suction. The portion of the foot that moves 

 forward is the elevated region," pp. 143 and 144. Locomotion in 

 the holothurian Stichopus panimensis Clark is effected by direct 

 monotaxic waves (Parker, '21). That is, "Creeping is accom- 

 plished in part by a muscular wave that originates at the posterior 

 end of the animal and sweeps over it to the anterior." It is inter- 



