194 THE FLOATING ANIMALS 



irregular figure in three dimensions, with length, 

 breadth, and thickness. Unfortunately, in the present 

 state of knowledge, we are unable to draw this figure 

 with certainty for any single species ; hence it will be 

 readily understood how important it is always to 

 record the temperature of the water from which a haul 

 is made, and, if practicable, also the salinity. 



One interesting result of the ability of many animals 

 to live below the surface, provided that they keep above 

 their minimum temperature, is that some arctic species 

 are continuously distributed from pole to pole in the 

 deep cold water of mid-ocean ; but have never been 

 captured anywhere near the surface in the tropics, where 

 the temperature is above their maximum. 



It has been stated (and it appears to be true for great 

 lakes, and to some extent for shallow water such as the 

 English Channel) that the greater part of the Plankton 

 comes to the surface only by night, and sinks to some 

 depth by day as a protection against too bright light. 

 Like many other accepted statements, this stands in 

 need of accurate testing as regards the open ocean. 

 Anyone becalmed in a sailing ship, or hove to in a yacht, 

 could gather valuable information on this point by 

 systematic day and night surface hauls in or near the 

 same position. Still more valuable information would 

 be yielded by similar day and night hauls at different 

 depths. This opens up two new questions how to 

 get a basis for comparison of different hauls, and how 

 to deal with subsurface hauls which are hardly deep 

 enough to rank as mid-water hauls. 



Hauls may be readily compared if standardized 

 that is to say, if they are made with nets of similar 

 mouth, length and mesh, for the same time, and at the 

 same speed of towing. In this case all that is necessary 

 is to preserve the whole catch, and to count the 



