• PEDAL LOCOMOTION IN ACTINIANS 123 



apparently for no more mechanism than is found in the relatively 

 simple neuromuscular structure of this organ acting on the fluid 

 retained under shght pressure in the adjacent body spaces. 



V. SUMMARY 



1. The direction of creeping of actinians is independent of 

 their secondary axis. In a single specimen of Actinia or of 

 Sagartia the direction of creeping may change from time to time 

 without relation to the secondary axis. The bilaterality of ac- 

 tinians, therefore, is not locomotor as in most animals. It is 

 probably respiratory. 



2. Actinian locomotion is accomplished by a wave-Uke move- 

 ment which progresses over the pedal disc in the direction of 

 locomotion. 



3. In a specimen of Sagartia with a pedal disc about 4 mm. 

 in diameter, the locomotor wave coursed over its disc in an aver- 

 age time of 1.65 minutes and with each wave the animal pro- 

 gressed on the average 1.2 mm. In a specimen of Condylactis 

 with a pedal disc 130 by 80 mm., the passage of a locomotor 

 wave required on the average 3 minutes and the animal pro- 

 gressed for each wave on the average 11.4 mm. 



4. In the actinian locomotor wave each point on the pedal 

 disc is successively raised from the substratum, moved forward, 

 and put down. 



5. The attachment of the actinians studied to the substratum 

 is due chiefly to adhesion heightened by the secretion of a thick 

 slime rather than to a sucker-Uke action of the pedal disc. 



6. The mechanism of locomotion consists of the circular mus- 

 cle of the pedal disc, the basilar muscles, and the longitudinal 

 muscles of the mesenteries, all of which act on the fluid-filled 

 spaces in the pedal region of the actinian. The pressm'e thus 

 generated is not above that of 6 cm. of water. 



7. Creeping can be accomphshed by an actinian from which 

 the oral disc has been cut away. Hence the pedal portion of 

 actinians, hke their tentacles, contains a neuromuscular mechan- 

 ism sufficient for its own activity. 



