18 PHYSIOLOGICAL CLASSIFICATION 



appear, as in Birgus latro, the coconut robber 

 crab, where there is an accessory pulmonary 

 chamber above the gills, and in Ampullaria, a 

 tropical freshwater operculate snail living in tanks 

 and swamps exposed to drought, where there 

 is an analogous arrangement. 1 An interesting 

 and rather subtle change has taken place in the 

 habits of typical land leeches in that they have 

 lost the power of swimming, having become 

 entirely stereotropic. 



Purely aquatic creatures are those which 

 breathe by absorbing the oxygen dissolved in 

 the water, and they are sometimes called "water- 

 breathing" animals on that account. If kept in 

 a limited volume of water the oxygen would 

 become exhausted and the animals asphyxiated 

 unless the water was frequently renewed as it 

 is in an aquarium. Asphyxiation sometimes 

 takes place under natural conditions in low-lying 

 swamps when the water in the deeper places 

 becomes stagnant after a prolonged drought. 

 The mud-dwelling fishes at the bottom may then 

 be seen swimming spasmodically towards the 

 surface to gulp in atmospheric air, often too 

 late to recover from the poisonous effect of the 

 noxious gases below. 



There are many partially or completely aquatic 

 animals which find their food under water and 



1 Birgus and Ampullaria are figured in Sempers " Animal Life," 

 London, 1890. 



