220 THE SEAS 



round to supply the various parts of our bodies. But in 

 the sea we never see fish coming to the surface to take in a 

 gulp of air as the marine mammals, such as whales, do. 

 They have no need to do so, for oxygen is everywhere 

 present dissolved in the surrounding water and fish have 

 a special apparatus, the gills, for extracting it. All the 

 larger marine animals have gills or some such specialized 

 structure for this purpose. But in the case of the very 

 small animals, in which the area of the body surface is 

 very large compared with the actual volume of the body 

 itself, oxygen can be absorbed anywhere over the surface 

 of the body. 



There is probably no part in the open sea where oxygen 

 is not present in solution in sufiicient quantity to support 

 a large number of animals. Isolated cases are known 

 however where there is no oxygen. Such conditions are 

 to be found in the deeper waters of the Black Sea. Here 

 we have a layer of light surface water down to about a 

 hundred fathoms, below which is heavy water. The upper 

 layers are of low salinity owing to the fresh water brought 

 down by the Danube and other rivers, but the deeper 

 waters have a high salinity ; hence the difference in weight 

 of the waters. Thus there is at a certain depth a layer 

 where the light surface water and the deep heavy water 

 meet. This forms a kind of boundary layer between the 

 two, and the waters above and below this depth do not 

 mix one with another. Therefore although the oxygen 

 supply in the upper layers may be very high, there is no 

 means by which this gas can be transported into the deeper 

 layers. As a result there is no oxygen below about a 

 hundred fathoms, the only gas present being the stinking 

 sulphuretted hydrogen. No living animals are therefore 

 present below this depth. The only organisms that can 

 live are certain bacteria that do not require oxygen, and 



