460 



THE BIOLOGY OF MARINE ANIMALS 



It is in these animals, again, that modifiability of response and learning 

 reach greatest development. 



The behaviour of shore crabs, notably Uca, reveals various interesting 

 social features, such as defence of territory, visual and auditory signalling. 

 Carcinus maenas, when tested in a simple maze, shows some ability to 

 improve with practice. Experiments with other crabs — hermit crabs, 

 fiddler crabs — have involved reversal of some proclivity. Fiddler crabs, for 

 example, learn to reverse the direction they normally take in escaping from 

 a particular situation. Good performances have been secured after some 

 thirty to fifty trials in some animals (99). 



The octopus has proved to be a most rewarding animal for experimental 

 studies of learning, and an attempt has been made to correlate certain 

 aspects of its behaviour with central nervous organization. Octopuses 



Shock 







T T T T 



6 



Days 



8 



10 



12 



Fig. 10.25. Progress of Learning in the Octopus 



In the experiments the octopus was fed with crabs, and in some trials a white square 

 was presented with the crab. Whenever the octopus attacked the crab + white square, 

 it was given an electrical shock. After 2-5 days of training, the octopus learnt to dis- 

 tinguish between the two stimulus-situations (crab alone, hollow circle, and crab + 

 square, solid circle), i.e. to leave the crab alone when the white square appeared. The 

 graphs show the ratio of number of attacks to trials during the course of experimentation 

 (abscissae in days). (From Boycott and Young, (13).) 



inhabit niches from which they venture forth to attack prey of suitable 

 character and size. In an aquarium they attack living crabs which they 

 recognize by sight. By presenting a crab in various situations, it has been 

 possible to analyse certain aspects of octopus behaviour. When an octopus 

 is shown a crab together with a white signal square, and it is given an 

 electric shock after making an attack, it soon learns not to attack when 

 this situation appears again (Fig. 10.25). It continues, however, to attack 

 crabs when these are presented alone. Memory of the former situation, 

 whereby an octopus learns not to attack a crab shown with a white square, 

 persists for 2-3 days in the absence of reinforcing stimuli. 



The brain of the octopus consists of discrete anatomical lobes, severally 



