192 THE FLOATING ANIMALS 



which can stand arctic cold of 30 to 35 F. will not 

 be happy in the tropics with a sea temperature of 

 8o° to 90 F. (although such cases are said to exist) ; 

 but many marine animals seem to be influenced by 

 quite small differences in temperature. Thirdly, if there 

 be too little salt in the water — for example, in estuaries 

 or the Baltic — many species will not live there. Lastly, 

 the character of the Plankton alters a good deal with 

 the depth ; it is not every animal which can thrive in 

 absolute darkness, at a very low temperature, on such 

 casual food as falls from the surface layers towards the 

 bottom. 



This last statement opens up a fresh branch of the 

 subject. Plankton is by no means confined to the 

 upper layers of water, but has been captured even at 

 the greatest depth to which suitable nets have been 

 lowered (2,734 to 2,187 fathoms). Such nets, termed 

 " closing nets," are lowered to a known depth, with 

 the mouth closed ; they are then opened by mechanical 

 means, towed either vertically or horizontally for a 

 known distance, closed again, and brought closed to 

 the surface ; they therefore only contain animals from 

 the desired depth. This mid-water work has been 

 systematically attempted only of late years, and forms 

 a fascinating branch of study. Very little is known 

 at present about the Plankton of great depths, and 

 any yacht with a steam winch can do most valuable 

 work in this direction. A description of the necessary 

 outfit is rather beyond the scope of these pages, but 

 details can easily be obtained from the Challenger 

 Society. Many of the animals from great depths seem 

 to be confined to deep water, but some are known also 

 from the surface. 



Of one branch of Plankton we know almost nothing— 

 that is, of those invertebrates which, although capable 



