BIOLOGICAL EFFECTS OF THE TIDES 253 



later the worm is found ensconced with its body in contact 

 with the stone, fully extended, its cirri properly expanded, 

 and looking altogether more comfortable. In other words, the 

 animal is markedly stereotaxic, and requires to have the nerve- 

 endings in the skin pressed against a solid surface. This is 

 not an isolated case ; one could illustrate the same phenomenon 

 from a number of different groups belonging to the shore or 

 elsewhere.' The wide-spread occurrence of sterotropism has 

 not escaped the attention of biologists. Willey,^ for instance, 

 regards it as something peculiarly fundamental, as a very 

 primitive property of living matter of which — and this is impor- 

 tant — a permanently sedentary habit is merely an extreme 

 manifestation. If this last surmise of Willey's is correct, 

 it follows that if we regard the sedentary habit among animals 

 as being primarily a means of resisting dislodgment by wave- 

 action, then we must also attribute these stereotropic phenomena 

 to the same agency. That is precisely what we mean to suggest, 

 viz. that this habit of maintaining the body in contact with a 

 solid surface may have its origin in the necessity for main- 

 taining a firm hold on the substratum under the influence of 

 wave-action, that it is, in fact, one of the lessons that was early 

 learnt in the hard school of the shore. 



Continuing to focus our attention on the wider biological 

 aspect, let us now pass to another aspect of the tides : that of 

 exposing the area covered by their action to alternate hydration 

 and desiccation. 



We know that the main route followed by animal life, in 

 its conquest of the land, has been from the sea by ways of rivers ; 

 but it is important to remember that this is not the only route 

 which has been taken by evolving life. The passage from sea 

 to land has also been made via the shore. Everyone knows 

 the common wood-louse, which lives in damp, decaying wood 

 or in humus. This animal closely resembles, and, in fact, is 

 closely related to, the essentially marine forms which we know 

 as " sea-slaters." Bridging the gap between the purely terres- 

 trial wood-louse and entirely marine Isopods are a number of 

 amphibious forms, one of which, Ligia oceanica, sometimes 

 arouses the curiosity of summer visitors to the seaside by 

 extending its area of activity to the promenade. Again, 

 there is a common British species of periwinkle, or sea-snail, 

 Littorina neritoides, which lives in crevices well above high-water 

 mark of spring-tides, and is satisfied with an occasional spraying 

 by the waves or with the moisture arising from the sea. A 



^ For further examples of stereotropism in animals reference should be 

 made to Loeb's Forced Movements, Tropisms, and Animal Conduct (Lippin- 

 cott, 1918). 



2 Willey, A., Convergence in Evolution. (London: Murray, 191 1.) 



17 



