BEHAVIOUR OF SHORE ANIMALS 223 



movements of the separate arms of the starfish, for instance, 

 is evidently the nerve-ring. It is easy to demonstrate this 

 experimentally. Thus, in spite of the absence of any brain, 

 or of anything representing a head region, a starfish invariably 

 moves in a determinate direction with one arm generally 

 directed forwards. This is brought about by the tube feet 

 of all five arms swinging in a direction parallel to that in 

 which the animal is moving, and without reference to the 

 long axis of the arm to which they belong. If, now, the 

 connection of each of the radial nerves with the nerve-ring 

 be severed, the tube feet of each arm will act perfectly 

 independently and the animal will be pulled first one way 

 and then another, according to which arm chances to exert 

 the strongest effort. 



A similar co-ordination of activities is particularly well 

 seen when a starfish is placed upon its back. The tube feet 

 are first extended in all directions and quite independently. 

 When, however, a hold is obtained by some of the tube feet, 

 the remainder at once exert their effoit in the same direction, 

 and the harmonious efforts of all the tube feet enable the 

 animal to turn over. 



The sense cells scattered through the ectoderm of star- 

 fish and sea-urchin render these animals sensitive over their 

 whole surface. The sensory elements are, however, particu- 

 larly numerous round the discs of the tube feet, and it is 

 these which form the chief sense organs. Each tube 

 foot also receives a branch of the radial nerve cord. The 

 tip of the terminal tentacle of each arm is modified as an 

 optical organ, consisting of a number of cup-shaped depres- 

 sions of epithelium, lined by pigmented, rod-like sensoi7 

 cells. The eyes of Asterias rubens and Solaster papposus 

 are said to perceive distant illumination which is in contrast, 

 either positively or negatively, to that of its immediate 

 surroundings. There is, however, no evidence of image- 

 forming or detection of movements on the part of the eyes 

 (Plessner, 191 3). According to another worker (Mangold, 

 1909), the use of the " eyes " in starfish is doubtful, their 

 excision making no difference to the light reactions. This 



