THE MOVEMENTS OF SHORE ANIMALS 157 



loose sand, when the exertion of even a small pull would at 

 once bring away the sand-grains to which the sucker is 

 attached. Jennings' explanation is that the action of the 

 tube feet is essentially similar to that of the leg of a higher 

 animal. In walking, each tube foot is extended forward, in 

 the direction in which the animal is moving, the angle the 

 tube feet make with the ray depending on the position of 

 the ray relative to the direction of movement. The tip of 

 the foot is then placed against the surface, the sucker helping 

 to give it firm attachment, and the whole tube foot is given 

 an impulse to swing back. Since the tip is against a solid 

 body and does not slip, this backward impulse results in 

 moving the body forward. The tube foot thus acts as a 

 partly rigid lever for pushing the body forward, and not as 

 a rope for hauHng. Apparently, according to Jennings, the 

 adhesive action of the tube feet only becomes of primary 

 importance when the starfish is climbing a steep surface or 

 hanging in an inverted position. 



Forms such as Astropecten which live and burrow in 

 sand have no suckers, and are unable to cHmb vertical 

 surfaces ; in view of the nature of the habitat this is, how- 

 ever, not necessary. They move by means of the arms, 

 which are used as shovels (Delage, 1903). 



Gemmill (19 12) has shown that locomotion in sea-urchins 

 may be achieved by means of the lantern (aided by the 

 spines), the animal raising itself on the tips of its teeth, 

 which, pressing hard against the supporting surface, result 

 in the accomplishment of a forward step or, rather, lurch 

 After each step the lantern is retracted and swings forward 

 so as to bring the teeth into position for initiating a new step. 

 By allowing the sea-urchin to travel over a surface spread 

 with a thin layer of plasticine Gemmill has obtained interest- 

 ing records of the mechanism of locomotion (see Plate XIII). 

 The lantern is employed mainly when the animal is out of 

 water, but it is sometimes of use during partial or complete 

 submergence in travelling up a slope or in mounting rapidly 

 a vertical (rough) surface. In particular, according to 

 Gemmill, the employment of the lantern during partial 



