178 THE BIOLOGY OF THE SEA-SHORE 



the environment. Thus, Bohn (1909) finds that " homing " 

 is not always certain. The movements of the animal seem 

 to be influenced by gravity. There are lines of least resist- 

 ance on the rock which are followed very much as one might 

 follow them in a forest. It is not necessary to invoke a 

 muscular or visual memory. 



Retrospect. — One of the most obvious features of the 

 shore fauna is the frequency of the sedentary habit, almost 

 certainly related to wave action. Among freely moving 

 types the methods of locomotion are, however, extremely 

 varied, this, in turn, being due to the varied nature of the 

 substratum. Owing to the relatively much greater density 

 of water as compared with air the degree of structural adapta- 

 tion required for an aquatic animal to lift itself from the 

 ground is much less than is required in the case, say, of a 

 bird. 



The chief methods of locomotion may be described as 

 follows : clambering, gliding, swimming, jumping, hanging 

 from the surface film. Locomotion in the starfish is of 

 particular interest since it represents a highly specialised 

 method of progress in relation to broken rock-surfaces. 

 Burrowing is extremely common, and enables a shore animal 

 to elude enemies and to escape desiccation at low tide. Every 

 stage of the burrowing habit may be observed, from tem- 

 porary refuge beneath a shallow layer of sand, as in shrimps 

 and prawns, to the boring of wood and rock involving a 

 very high degree of specialisation. 



Migrations are usually of small extent and have been 

 insufficiently observed. There is evidence of a " homing 

 sense " in the common Umpet and its relatives. 



