96 A BIOLOGY OF CRUSTACEA 



shaken gently all the individuals swim rapidly downwards as if to 

 escape. When the jar is allowed to stand still for a while the water 

 fleas gradually swim upwards again and occupy all the water. A 

 further shaking sends them scurrying downwards again, but it is 

 noticeable that a few continue swimming more or less normally. If 

 this is repeated at regular intervals, with about the same degree of 

 shaking, the response wanes, and very few individuals swim down- 

 wards when the jar is shaken. This might be interpreted as a form 

 of fatigue, but the response reappears if the shaking is made more 

 violent, or if the creatures are stirred around with a glass rod. 



Habituation is very necessary in the natural world. If an animal 

 responded to all the possible danger signals it would rapidly become 

 neurotic and would have no time for feeding or other activities. 

 ■ Crustacea also show more complex types of learning. Asellus 

 aquaticus can be taught to choose between a left or a right turn in 

 a simple T-shaped maze. The training involves punishment when 

 the wrong turn is taken; this need not be severe, a light touch from 

 a paint brush is sufficient. Woodlice can go one better and choose 

 between an upper and a lower path, a feat that Asellus seems 

 incapable of learning. Crabs and lobsters can be taught mazes with 

 several turns. 



It has been claimed by one worker that Daphnia can be taught 

 to swim through a tube towards the light, but other workers have 

 found no signs of such ability in the Cladocera. 



Another type of learning is shown by crabs when kept in 

 aquaria; they quickly learn that the appearance of a person is 

 associated with feeding time, and they emerge from their hiding 

 places in attitudes that clearly indicate that they are expecting 

 food. Some crabs also show that they have a good memory of their 

 home territory by their ability to return to their own burrow or 

 crevice after foraging on the shore. 



" An instance of learning a new feeding habit was observed in the 

 Plymouth Aquarium by Dr. D. P. Wilson. Four large spiny lobsters 

 (Palinurus vulgaris) were kept in a tank with twenty or more hermit 

 crabs which were carrying whelk shells. These two types of Crus- 

 tacea lived together quite happily for several months, until, during 

 a period when food was scarce, the spiny lobsters attacked the 

 hermits, dragged them out of their whelk shells, and ate the soft 

 abdomens. The spiny lobsters persisted in this habit even when 

 provided with other food; eventually they had to be removed and 

 replaced by others which had not acquired a taste for hermit crabs. 



