230 THE BIOLOGY OF THE SEA-SHORE 



(2) moisture, (3) pressure or contact, and (4) gravity. These 

 are obviously not peculiar to the sea-shore, but, owing to the 

 transitional and fluctuating character of this area, they act 

 and interact in a manner which is perfectly distinctive and 

 hardly paralleled elsewhere. 



Thus, to cite an example, the behaviour of a periwinkle 

 at a particular moment with regard to light will depend upon 

 the degree of hydration of its tissues, and in certain cases 

 a complete reversal of the behaviour towards light may be 

 brought about simply by placing the animal beneath a jet 

 of water (Bohn, 1905). 



The Reaction 0! Shore Animals to Light. — The import- 

 ance of light to the life of the organism is emphasised by 

 Benjamin Moore (1909), who says that " since the very 

 existence of a living organism, either directly or indirectly, 

 is dependent upon the energy of light and the transformation 

 of this into other types of energy, it is not surprising that 

 reactions to light are among the most fundamental and most 

 widely spread throughout the whole world of organised 

 living creatures. Such reactions must have been developed 

 in the very beginning of the dawn of life when the first 

 living cells commenced to synthesise organic products from 

 the inorganic materials of the environment by the use of the 

 store of energy from the sunlight. Later on, organisms 

 arose which were only dependent upon the light at second 

 hand, since they were able to consume the synthesised organic 

 products formed by other organisms converting the light 

 energy directly, and so were only indirectly dependent upon 

 the light for their existence. Even for this type of organism, 

 utilising the light energy indirectly, reactions to light 

 remained essential in the search for food and for other 

 physiological functions, and also there would be an inherit- 

 ance of relationships to light derived from the earlier 

 ancestry with direct dependence upon light." 



" At a later stage structures or organs arose specially 

 adapted for light reactions, and in those living creatures 

 possessing such organs there probably came a deterioration 

 of the sensitiveness to light of the remaining cells of the 



