30 



The Selection Theory 



serve tlie 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 (5) of Stjnapta lappa. Length of anchor = OSo mm. 

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



Fig. B. 

 Anchor (a) and basal-plate (6) in side-view (after Oestergren). 



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

 towards the ground wlien the corresponding part of the body 

 thickens, and returning to the original position at an angle of 4.5° to 

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

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



