30 



The Selection Theory 



serve the footless Synapta as auxiliary organs of locomotion, since, 

 when the body swells up in the act of creeping, they press firmly with 

 their tips, which are embedded in the skin, against the substratum 

 on which the animal creeps, and thus prevent slipping backwards. 

 In other Holothurians this slipping is made impossible by the fixing 



Fig. A. 



Anchor (a) and basal-plate (b) of Synapta lappa. Length of anchor = 0-35 mm. 

 (After Oestergren, Zool. Anzeiger, xx. 1897.) 



Anchor (a) and basal-plate {b) in side-view (after Oestergren). 



of the tube-feet. The anchors act automatically, sinking their tips 

 towards the ground when the corresponding part of the body 

 thickens, and returning to the original position at an angle of 45° to 

 the upper surface when the part becomes thin again. The arms 

 of the anchor do not lie in the same plane as the shaft, and thus 



