70 THE BIOLOGY OF THE SEA-SHORE 



cling in like fashion to the rocks, and thus resist the force 

 of the waves. This method of adhesion must be a great 

 convenience to these shallow - water dwellers in rough 

 weather, when the swirl of the surf would otherwise dash 

 them ashore, or, at any rate, transport them far from their 

 eggs. Other fishes, not provided with these suctorial fins 

 or discs, have to burrow in the sand." According to experi- 

 ments by Cunningham (Aflalo, op. cit.), the fish by pumping 

 water with its fins between its body and the object of attach- 

 ment sets up differences in the pressure and direction of the 

 water which are sufficient to keep it in position. 



3. Form of Body adapted to minimising Friction. — There 

 is no fact more evident on the shore than that the animals 

 which compose its fauna are essentially flattened forms. 

 Everywhere the need for reducing friction is apparent. We 

 have encrusting sponges, like Halichondria and Oscarella, 

 leaf-like Turbellarians, leaf-like or encrusting Polyzoa like 

 Flustra and Membranipora, encrusting compound Tunicates, 

 as Botryllus. Among plants, the encrusting stony Litho- 

 thamnion, Melohesia, and others have successfully solved 

 the problem presented by the surf. The numerous molluscs 

 are all squat forms, their shells devoid of spines and mostly 

 inornate. Shore Crustacea show the same influence, the 

 flattening being either lateral as in Amphipods like Gam?naruSy 

 or dorso-ventral as in Isopods like Ligia and in the crabs, 

 especially Porcellana platycheles. 



The " stream-line " form, however, is less common than 

 one would expect at first glance. This is probably because 

 shore forms are mostly sluggish in habit, and more par- 

 ticularly because the impact of the water is not acting, as in 

 a river, in one definite direction. Moreover, the majority 

 of shore animals are clinging to and moving upon solid 

 surfaces so that they have to meet the water on one side only. 

 If, as Needham and Lloyd {op. cit.) remark, we take the typical 

 fish form and divide it in the median vertical plane and then 

 apply the flat side to the supporting surface, flattening it out 

 a little at the edges, the form obtained is that of a limpet. 

 This shape is very common on the shore and is directly 



