The Brain and What It Does 123 



Similar and probably much simpler involuntary responses to 

 stimuli are quite common even in very primitive kinds of animals. 

 Some organisms will move towards the light, others will move into 

 shaded regions. For example, some hydrozoa, like Gonionemus, 

 prefer to be in the shaded parts of an illuminated tank; others, such 

 as Englena viridis, seek the lighted parts. The mollusc, Pholas, in 

 which the whole surface is said to be light sensitive, retracts its 

 syphon in response to light, while in Ciona intestinalis, light causes 

 a closure of the syphon apertures. In sea urchins the spines react to 

 variations in light, e.g. in Diadema antillarum, spines are moved 

 vigorously towards regions shaded by objects.^ 



Worms like Branchiomma have light-sensitive tentacles, and the 

 passage of a shadow over these tentacles causes it to withdraw into 

 its tube. Starfish often have light-sensitive patches (or primitive eye- 

 spots) on the tips of the rays. 



Many organisms react to the presence of chemical substances, 

 like oxygen. These are all automatic responses to stimulation which 

 act directly on the organism's means of propulsion. 



But in all the more developed forms of animal life, something 

 more than simple reflex action is required. The animal must not only 

 receive sense impressions; in order to make use of them, it must be 

 able to make some sort of an interpretation of them. The interpreta- 

 tion is based on experience, i.e. on the results of previous actions in 

 similar situations. The animal learns from experience, and he could 

 not learn if he did not remember. 



This is an entirely new thing in the world of life; a totally different 

 mode of response to stimulation and one which has turned out to be 

 enormously more effective. It is, in fact, the first beginnings of 

 mentality since the animal learns something about his surroundings 

 and uses, or tries to make use of, that knowledge. 



Very many experiments have been made of this learning process. 

 Professor J. Z. Young has experimented with octopuses. They were 

 offered either a crab or a crab with a white plate behind it. The 

 animal lived among some bricks at one end of a tank and the crab 

 was lowered by a thread at the other end. As soon as the moving 

 crab appears, the octopus swims or walks across the tank and throws 

 itself over the crab, gathers it up and returns with it to its home. 

 When the crab with the white plate was presented, an electric shock 

 was applied as soon as the octopus had seized the crab. This caused 

 it to go back to its home quite quickly and it very soon learnt to 

 associate this unpleasant experience with the white plate so that, 



1 N. Millott, Endeavour, XVI, 19 (1957). 



