114 G. H. PARKER 



position and the actinian after a short interval began again to 

 creep away from the light and without changing the direction of 

 its secondary axis. Three other specimens were tested in essen- 

 tially the same manner and gave similar results. In all cases 

 new courses of locomotion in close agreement with the direction 

 of the light were established in from 10 to 15 minutes after the 

 change in the position of the light was made and the animals 

 could thus be induced to creep at any angle to their secondary 

 axis. In other words, locomotion in actinians is radial in char- 

 acter and is not limited by the structural bilaterality of these 

 forms. This bilaterality, then, is not locomotor, as it is in most 

 of the higher animals, but must be associated with some other 

 general function. What this function is cannot be stated with 

 certainty, but the fact that the bilaterality of the sea-anemones 

 centers around the mouth which, though a single opening, is 

 elongated and differentiated for simultaneous incurrent -and ex- 

 current streams, and thus serves respiration, the appropriation of 

 food, and the discharge of excrement, suggests that these rather 

 than locomotion are some of the activities whose influences have 

 brought about the bilateral symmetry of the sea-anemones. 



III. MECHANICS OF PEDAL LOCOMOTION 



In all the actinians I have studied, locomotion was accom- 

 plished by a wave-like movement that passed over the pedal 

 disc. This would begin at the rear edge of that organ and pro- 

 ceed thence to its front edge. Such movements are almost al- 

 ways accompanied by an elongation of the pedal disc in the di- 

 rection of locomotion as observed by Osburn ('14, p. 1165). In 

 Metridium these waves have been briefly described and figured by 

 McClendon ('06), who ('11, p. 61) has also identified them in 

 Cradactis. So far as I am aware, however, the details of this 

 movement have never been very fully investigated. 



In a specimen of Sagartia whose pedal disc had a diameter of 

 about 4 mm. the waves could be seen coursing from the rear to 

 the front as the animal crept over the level bottom of a glass 

 dish. In 15 minutes 5 successive waves had passed across the 

 disc and the animal had crept about 6 mm. The slowest wave 



