MONITORS 225 



circulation. Then our proprioceptors tell us that we are 

 feeling refreshed and vigorous again. 



Some very recent experiments have shown that certain 

 animals have senses of which we have no conception by 

 analogy with our own. A kind of prawn is able to respond 

 to differences in the pressure of water produced by differences 

 in depth of only a few centimetres. The surface membranes 

 of all cells are electrically charged and the charges on the 

 cells in different parts of the body are often different. When 

 such differences in electric potential occur on different parts 

 of the bodies of animals that live in water an electric current 

 flows from one to the other. Although the current is extremely 

 minute in the prawn it is sufficient to produce some electro- 

 lysis so that an excessively thin layer of gas covers parts of 

 the body, too thin to be seen through any ordinary micro- 

 scope. Water is incompressible and a prawn's body, per- 

 meated everywhere with fluid, consists of over 90 per cent 

 of water so that any change in pressure is distributed evenly 

 throughout it. Thus no differences of pressure are produced 

 that could act upon a sense-organ. But the minute film of 

 gas is compressible, and the appropriate sense-organs of the 

 animal can respond to the alterations in the pressure of the 

 gas. The animal is thus able to respond to differences in 

 absolute pressure, in contrast to the transient differences in 

 pressure to which fishes respond through their lateral-line 

 organs. It is difficult to decide whether this sense is really 

 proprioceptive or exteroceptive — a response to internal or 

 external stimuli. 



And what of the migrations and homing abilities of birds 

 and other animals? Much thought has been devoted to the 

 subject of bird navigation but, without in any way decrying 

 the careful and brilliant research that has been devoted to 

 the subject, I cannot help thinking that we are as far from 

 a solution of the problem as ever. The facts of bird migration 

 are well known, including the migration of young cuckoos 

 which fly off unerringly to their winter quarters long after 

 the adults have left our country, and which have, as far as 

 we know, nothing but their innate instincts to guide them. 

 Many other animals are expert at homing. Even the limpet 

 on the surf-beaten rock prowls about at high tide grazing on 



