THE IMPENETRABLE SEA 



Relaxing and contracting these mantle muscles, which 

 bathe its gills, jets of water pass into and emerge from the 

 cavity. Using these streams of water the animal breathes 

 and moves. When lazily resting or moving slowly through 

 the water, the mantle circulation is gentle, rhythmic and 

 slow. But when alarmed or excited the mantle muscles 

 work rapidly, driving out jets which give the creature 

 quick and easy movement. When the squid points its 

 funnel forward it moves rapidly backwards, and when 

 it wants to move forward it turns its funnel back on 

 itself. 



The funnel then shoots streams of water backwards 

 past its moving body, sending the animal forwards 

 quickly in bursts of speed. Cephalopods like the octopus 

 and squid are not the lazily moving creatures that they 

 are often thought to be. The speed of an octopus, swim- 

 ming steadily, has been timed as about the same as a 

 human swimmer — four miles an hour, but they can dart 

 about like lightning when chasing their prey or eluding 

 their enemies. 



The quick movements of these creatures are due to the 

 high development of its giant nerve-fibre system. In the 

 mantle are comparatively large nerve-fibres as well as 

 numerous smaller ones, and down all these fibres pass the 

 inconceivably swift impulses from the brain which gal- 

 vanize the muscles into action. So well-developed is the 

 cephalopod's nervous system that reactions to external 

 stimuli are far quicker than those of many other sea 

 animals. 



The suckers along the arms of the cephalopods are 

 marvellous examples of natural mechanisms. Each con- 

 sists of a muscular membrane, reinforced around its rim 

 in some species. The centre of each sucker operates like a 

 piston, so that as it is raised a partial vacuum is created 

 within the sucker giving it a powerful grip on anything to 

 which it is attached. 



Carrying out tests with a spring balance, G. H. Parker 



250 



