66 THE BIOLOGY OF THE SEA-SHORE 



force. In these circumstances no animal could possibly 

 survive which had not some efficient means either of eluding 

 or withstanding the impact. And so it is not surprising to 

 find that wave-shock has had a profound influence on shore 

 animals, particularly as regards their form and habit. In- 

 deed, granted the littoral origin of life, it seems not too much 

 to consider the sedentary habit as due essentially to this 

 factor. 



To a lesser extent the problem of wave-impact occurs on 

 the margins of fresh-water ponds and lakes. The special 

 adjustments of fresh-water organisms to withstanding the 

 wash of moving waters have been studied by Wesenberg 

 Lund (1908), and by Needham and Lloyd (1916). They 

 show an interesting parallelism to those occurring among 

 shore animals. 



In considering the question of wave-impact in relation 

 to shore animals it will be useful to follow Hunt (1885), and 

 to distinguish between : 



(a) Animals living on rock or other firm foundation ; 

 '{b) Animals living in sand or other unstable deposit ; 

 and to deal with them separately. 



(a) Animals living on Rock or Other Firm Foundation 



Forms inhabiting areas of this type have often the 

 advantage of natural cover in the shape of sea-weed, the part 

 played by the latter in absorbing wave-shock being of some 

 importance. This is particularly true of Ascophyllum nodo- 

 sum, the long (often several yards) strands of which have 

 an undoubted effect in smoothing out the waves (see 

 Plate X). Apart from this the following are some of the chief 

 ways in which the problem is met by this class of animal. 



I. Occupation of a Sheltered Position. — It is a character- 

 istic of shore animals that they find shelter in nooks and 

 crannies, under stones and seaweed and are rarely found in 

 exposed situations. On a bad coast, like that of Cardigan 

 Bay, if the strike of the rocks is fairly parallel with the 

 coast, the dip of the rocks becomes an important factor as 

 regards the fauna. Where the dip of the rocks is low, the 



