BEHAVIOUR OF SHORE ANIMALS 235 



represented by high and low spring tides. Within much 

 narrower limits (owing to moisture-conserving arrangements, 

 etc.) the amount of water in the tissues of shore animals 

 also varies, and since we know that water is of fundamental 

 importance to vital processes, it is only to be expected that 

 its presence in varying quantity should have considerable 

 effect on behaviour. We have already had evidence of this 

 in the completely different phototaxic responses of Littorinas 

 before the tide reaches them and when it has just left 

 them. 



Anna Drzewina (1908) has noticed that shore crabs 

 {Carciniis moenas) living under stones at the water's edge 

 show a very definite response to the moisture arising from 

 the sea, which may be regarded as a kind of hydrotaxis. 



A shore crab placed on the ground 50, 100, or 200 metres 

 from the water's edge invariably tends to make a direct 

 line towards the sea, in spite of any obstacles it may encounter, 

 and practically regardless of wind, shadows, and degree of 

 slope. After rain, however, when moisture is rising from 

 the ground as well as from the sea, this behaviour is no longer 

 manifested. It is particularly evident among those crabs 

 which live on a shore where the sea retreats a long way and 

 where desiccation may be severe. 



In other habitats crabs do not behave in this way. For 

 instance, those which live under stones near low- tide mark, 

 and are in no danger of drought, exhibit this response either 

 very slightly or not at all. The danger in this case is that 

 of being carried away by the waves. At a certain point of 

 the French coast (Gatteville lighthouse) the seaweed-clad 

 rocks project far into the sea and are pounded by the waves. 

 Numbers of crabs are found clinging to the weeds, and no 

 sooner are they detached than they return to their original 

 position. When placed upon a sandy beach, instead of 

 proceeding towards the sea, they make for rocks covered 

 with FucuSy if these are near, or towards any large expanse 

 of similar colour, in this case being guided, apparently, 

 by their powers of vision (Bohn, op. cit.). 



Reactions to Pressure or Contact. — Certain animals 



