FRESH WATER ANIMALS 207 



they live. Both systems are practicable in the quiet of fresh 

 water, but neither is adapted for existence in the sea. An in- 

 sect carrying air outside the body is likely to lose this surplus 

 if subjected to wave action, while the varying pressures on the 

 sea shores, due to waves and tides, prevent the existence there 

 of insects with a closed air reservoir. Free swimming insect 

 larvae, either gill breathing or taking air from the surface, can- 

 not exist at sea for the wave motion, acting as a barrier between 

 the water and the air, prevents their transformation to adults. 



]Many insect larvae live in tide pools and salt marshes and 

 in the rotting sea-weed on the shores; anywhere, in fact, 

 where they are not subject to wave action or where the wave 

 action is only slight and discontinuous, showing that the salt- 

 ness of the sea is no deterrent to their existence in it; and I 

 have caught many giant water beetles, water boatmen, and 

 other t>pes of water insects in the tow-net far from land in the 

 summer when the sea was calm. 



The air-breathing frogs and toads and salamanders have no 

 ribs, or only very weak ones, and their lungs are therefore 

 subject to the full force of the water pressure. So far as I 

 know none of these are able to support an added pressure of 

 two atmospheres. Neither are any of them large and strong 

 enough to be able to secure a continuous supply of air in 

 stormy weather, and as we know them the gilled larvae are too 

 feeble to withstand the action either of waves or tides. 



The sea-cows, seals and whales, crocodiles, water snakes and 

 lizards are large and strong and have their lungs protected by 

 stout ribs and a thick covering of muscles which counteract 

 the water pressure. 



Certain gill breathing salamanders live under heavy water 

 pressure, as in artesian wells. But all gill breathing salamanders 

 are feeble creatures, few in numbers, and apparently able to 

 exist only in the absence of wave action and of serious com- 

 petition. They seem to represent a special t^^pe evolved in 

 fresh water, and even in fresh water able to live only in a very 

 circumscribed environment. 



