RESPIRATION IN SHORE ANIMALS 215 



have been attempted to test the survival value of a particular 

 character, and on that account we have more than the usual 

 amount of justification when we speak of the narrow frontal 

 breadth of shore crabs in Plymouth Sound as being an 

 adaptive character — adaptive, that is, to the high degree of 

 turbidity of the water which these animals are required to 

 breathe. 



Retrospect. — In spite of the fluctuating character of the 

 tidal area the great majority of shore animals are typical water- 

 breathers. Many of them, in consequence, are obliged to 

 become inactive at low-tide and to make shift with the small 

 amount of oxygen present in the moisture retained on the 

 surface of the gills. A small minority are truly amphibious 

 in the sense that they can carry on their activities in two 

 distinct media and have been actually found to breathe either 

 air or water as circumstances require. Usually, this dual 

 mode of breathing is accompanied by a certain amount 

 of structural modification, but this is not always the case. 

 The fact of the matter is that, physiologically, the difference 

 between air- and water-breathing is not profound, and in 

 one important feature the two processes are identical — the 

 membrane through which the exchange of gases is effected 

 has its surface bathed in moisture. There is a sense in 

 which the most typical land animals may be said to breathe 

 dissolved oxygen. 



The chief hindrance to the passage of an aquatic animal 

 from its original habitat to air is the tendency of the respira- 

 tory surfaces, if these are exposed, to become dried up. 

 This provides us with a reason why the amphibious habit 

 has proceeded furthest in forms such as crabs and snails 

 (and to a lesser extent in fishes), for these have their respira- 

 tory organs effectively protected. It by no means follows, 

 however, that the protection has been evolved for this 

 particular purpose. 



The habits of tube-building and of burrowing in the 

 sea-floor are other typical shore activities which have 

 influenced the function of respiration. Burrowing, in 



