232 THE BIOLOGY OF THE SEA-SHORE 



rocky shores of the English Channel behave differently as 

 regards light from those which live in sandy parts of the 

 Arcachon basin. The former move away from the light 

 into the shade ; the latter, since there are no shadows on a 

 flat shore, remain stationary in phototropic positions with 

 the tips of the arms turned inwards in such a way as to 

 protect them from the injurious influence of the direct 

 solar rays. 



Similarly with periwinkles, the sign of the phototaxis may 

 be reversed by the presence of moisture. Thus, when the 

 rocks where Littorinas occur are reached by the waves, the 

 animals , being negatively phototaxic , are guided by the darkness 

 to the deepest crannies where, of course, they find the 

 greatest degree of shelter. At ebb-tide, when the water 

 leaves the rocks, the animals leave their crevices and proceed 

 towards the light among the wet algae where they find their 

 nourishment. 



As we have already mentioned, a periwinkle which moves 

 away from the light can be made positively phototaxic by 

 the simple process of placing it under a jet of water (Bohn, 

 op. cit.). 



Bohn's numerous experiments with Littorinas would 

 seem to leave very little that is unpredictable in the behaviour 

 of these molluscs. Thus, among other points, they are said 

 to be attracted and repelled by stones and rocks, the extent 

 of the influence of these objects being dependent upon their 

 size and degree of illumination. 



Sedentary animals react to light much in the same way 

 as plants do. Since they are incapable of moving their 

 position they respond by growing or bending towards the 

 source of illumination. When, for instance, the Polychaet 

 Spirographis spallanzani, with the tough but flexible tube 

 in which it lives, is placed in an aquarium which receives 

 light from one side only, it is not long before the anterior 

 end of the whole tube is raised by the worm until the axis 

 of symmetry of the gills faces in the direction of the rays 

 of light. The animal retains this orientation as long as the 

 direction of the light rays remains unaltered. If, now, the 



