CHAPTER X 



Respiration in Shore Animals 



Notwithstanding the fluctuating nature of the tidal por- 

 tion of the sea-shore, the animals which live there are 

 practically all truly aquatic in their mode of respiration. 

 They require to breathe dissolved oxygen, whereas land 

 animals, of course, use the free oxygen of the atmosphere. 

 The proportion of oxygen in pure air is very nearly 21 per 

 cent, by volume. On the other hand, according to the 

 computations of Fox (1907), one litre of sea- water of average 

 salinity (say 34*33 pro mille) and temperature 10° C. will 

 absorb from a free dry atmosphere of normal pressure 

 6'44 c.c. of oxygen. It thus follows that, from the point of 

 view of oxygen supplies, land animals have a considerable 

 advantage over animals which live in the waters. 



The oxygen content of sea-water diminishes with 

 increasing temperature and salinity, and also with increasing 

 depth. Owing to their shallowness, relatively low salinity, 

 and to the effect of wave action, the littoral waters are the 

 most highly aerated of the whole sea — one of the reasons 

 why this area is so favourable to life. 



In both terrestrial and aquatic animals the respiratory 

 exchange is effected by bringing an adequate quantity of 

 the blood which, in all vertebrates and in most invertebrates 

 also, contains a pigment with a special affinity for oxygen, 

 into the closest possible relation with the ambient medium. 

 What we are accustomed to regard as the typical organ of 

 water breathers is a thin- walled, usually feathery, structure, 

 known as a gill, such as is present in fishes and the larger 

 crustaceans. But among invertebrates the term " gill " 



