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 
Anchor (a) and basal-plate (5) of Synapta lappa. Length of anchor = 0°35 mm. 
(After Oestergren, Zool. Anzeiger, xx. 1897.) 
Fig. B. 
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 
