THE MOVEMENTS OF SHORE ANIMALS 167 



closed by sand from the bottom or by falling sand as the 

 animal enters. In addition to burrowing, the " beach-flea " 

 can also run and hop but swims with difficulty. The 

 corresponding British species is the common Talitrus 

 orchestia. Another burrowing Amphipod, which is of very 

 common occurrence on sand and mud-flats between tide 

 limits in this country, is Corophium longicorne. 



Among Crustacea the burrowing habit has led, in many 

 cases, to numerous adaptations of structure in relation to 

 digging, and particularly in connection with the difficulties 

 of maintaining a respiratory current beneath the sand, 

 e.g. Corystes and Galathea (see under Respiration, p. 213). 



The great majority of bivalve molluscs are burrowing 

 forms. The form of the shell is obviously of great 

 importance in relation to this habit. Forms with com- 

 pressed, wedge-shaped valves usually burrow rapidly and 

 deeply and have elongated siphons. Globular forms like 

 the cockle burrow less rapidly and to a shallower depth, 

 their shape rendering them less liable to be dislodged by 

 wave action. Probably the most actively burrowing form 

 of all is the razor-shell (Solen). The habits of an American 

 species of razor-shell, i.e. Ensis directm, which is abundant 

 all along the east coast of the United States, have been 

 studied by Drew (1907). In burrowing, the foot is worked 

 into the mud, the end swelled to a knob, and by its sudden 

 withdrawal the shell is drawn to the position previously 

 occupied by the anchored end of the foot. At the same time 

 a strong jet of water is expelled from the anterior end of 

 the shell so that the mud is softened as the shell descends. 

 Similar observations have been made on the native species 

 Solen ensis by Elmhirst (19 10), who likens the fleshy expansion 

 of the foot to a mushroom anchor, and remarks on its suit- 

 ability for giving a firm grip of the sand. 



Boring.— As a specialised form of burrowing we have 

 the excavation of channels in wood and rock, a mode of 

 progression entailing perhaps more extreme modifications 

 of form than any other on the shore. From the economic 

 standpoint the damage done by wood-boring organisms 



