THE BENTHAL 



203 



known in which the secretion from the sole of Chiton pierced lead pipes 

 7 mm. in thickness. The form of the shell is also influenced by the 

 strength of the surf (cf. p. 152). 



At the level of the Fucus, the individuals of the edible mussel at- 

 tach themselves to the rocks with their byssus ; for the most part they 

 do not reach the laminarian zone. These mussels thrive best where they 

 are most exposed to wave action, tending to be scarce or absent on 

 sheltered cliffs and islands (Fig. 37) . 3G This is remarkable, because 

 Mytilus also thrives in places where the movement of water is rela- 

 tively slight, as in the brackish water at river mouths and in some 



Fig. 36 



Fig. 35. — Littorina rudis, upper, European seas, and Nerita Juniculata, lower, the 



Indian Ocean. Natural size. 



Fig. 36. — Shell of Patella xmlgata from the side, and outlines of the shell, from 



rough and smooth surfaces. After Russell. 



bays. It is an extremely adaptable animal, and one may accordingly 

 conclude that it is not the physical conditions but the biocoenotic re- 

 lations that condition its abundance. In the surf the individuals are 

 small but thick shelled and with a strong byssus. Where the wave 

 action is weaker, algal growths of Fucus gain the ascendancy over 

 them ; at transitional areas thin patches of Fucus are interspersed with 

 Mytilus colonies. At other places enemies, such as the starfish Asterias 

 and the codling, keep them in check, for example, in the eelgrass 

 meadows of the Danish fiords. 37 In the Bay of St. Malo, Mytilus has 

 disappeared entirely with the increase of the octopus. 38 The surf zone, 

 however, and also the brackish water, are closed to these predators, so 

 that the mussel can flourish there. It is not unlikely that the exclusion 

 of enemies and more favored competitors by wave action has operated 



