SPECIAL PROBLEMS OF SHORE LIFE 71 



traceable to the environment. Eliot (1910), in discussing 

 the close resemblance between Nudibranchs of very different 

 types, says " the general conditions of marine life probably 

 make it natural that a mass of soft flesh adhering by its 

 base, not moving much and not attempting to swim, should 

 assume this form." 



Interesting variations in form have been noticed among 

 molluscs, which bear a direct relation to the degree of exposure 

 (see Fig. 4). Individuals of Purpura lapillus, the common 

 dog whelk, " occurring in very exposed situations, e.g. Land's 

 End, outer rocks of Scilly Islands, coasts of North Devon 

 and Yorkshire, are stunted, with a short spire and relatively 

 large mouth, the latter being developed in order to increase 

 the power of adherence to the rock and consequently of 

 resistance to wave force. On the other hand, shells occurring 

 in sheltered situations, estuaries, narrow straits, etc., are 

 comparatively of great size with a well-developed, sometimes 

 produced spire and a mouth small in proportion to the area 

 of shell surface " (Cooke, 1895). 



Davis and Fleure (1903) claim that the shells of limpets 

 on exposed flat surfaces are typically low and broad, and that 

 shells on sheltered surfaces are typically high and narrow. 

 This conclusion, however, is only partly confirmed by the 

 observations of Russell (1907). 



{h) Animals living on Sand or Other Unstable Deposit 



The habitat being unstable no powers of attachment 

 will serve the animal, its safety depending practically 

 entirely upon the celerity with which it can penetrate the 

 sand. 



According to Osier, quoted by Hunt (1884), " animals 

 which from their small size or the little depth at which they 

 reside are particularly liable to be exposed, will generally 

 burrow readily, but the large species will scarcely attempt 

 to bury themselves except in the very young state." It is 

 not difficult to see that while some Lamellibranchs are 

 admirably adapted by their shape to penetrate the sand, 



