RESPIRATION IN SHORE ANIMALS 205 



on an active existence in both air and water ; how those 

 animals which are quite incapable of breathing atmospheric 

 oxygen survive the periods of low tide ; what is the effect 

 on respiration of such characteristic shore habits as burrow- 

 ing and living inside a tube ; what is the influence of wave 

 action and of low salinity upon this same all-important 

 function ? and so on. Some of these questions have been 

 answered already in connection with the discussion of the 

 ways in which shore animals meet the problems raised by 

 the retreat of the tide ; others are new and now claim our 

 attention. 



The Accommodation of Water-breathers to Breathing in 



Air. How is it that certain animals are able to carry on 



their activities both in and out of water, and what are the 

 most essential features which distinguish air- and water- 

 breathing ? In spite of the apparently great difference in 

 structure between a water-breathing organ and one adapted 

 to breathing air, the physiology of respiration in the two cases 

 is essentially similar. Thus, a point of great importance is 

 that in both cases the oxygen is absorbed from the sur- 

 rounding medium by a membrane which is kept moist and 

 which, as Semper {op. cit.) points out, should be able to carry 

 out its function quite equally well whether the oxygen is re- 

 ceived from air or from water. In fact, says Semper, provided 

 the osmotic power of the respiratory membrane remains the 

 same, since the amount of oxygen taken up within a given 

 period obviously depends on the proportion of oxygen 

 contained in equal volumes of the air or the water, the 

 respiratory surface may be in a position to take up more 

 oxygen from the air than from the water in the same unit of 

 time, because air has a larger admixture of free oxygen. 

 From this it follows that, if there is no other hindrance to 

 an alteration in the mode of life, an animal which has been 

 used to breathing in water will more easily accustom itself 

 to breathe in air than an animal living in the air, on the 

 contrary, can accommodate itself to breathing in the water. 

 One serious hindrance there is to the passage of an aquatic 

 animal to air : the tendency of the gills, if these are exposed, 



