CRUSTACEA OF THE SEASHORE 113 



at night ; and the interesting observation has been 

 made that Prawns kept in the dark continue for 

 three or four days to show a periodic expansion and 

 contraction of the chromatophores, corresponding 

 to the alternation of day and night. It seems that 

 the rhythm of light and darkness has become im- 

 pressed on the chromatophore system of the animal, 



FIG. 41 THE COMMON PORCELAIN CRAB (Ponellana longicornis), 

 SLIGHTLY ENLARGED, AND ONE OF THE THIRD MAXILLIPEDS 

 DETACHED AND FURTHER ENLARGED TO SHOW THE FRINGE OF 

 LONG HAIRS 



and the movement of the pigments is regulated by 

 something analogous to memory. 



It has already been mentioned, in dealing with the 

 Lobster, that certain Crustacea have the power of 

 voluntarily throwing off some of their limbs (auto- 

 tomy). In many cases, as in the Lobster, this power 

 is mainly of use in enabling the animal to discard an 

 injured limb ; but there are some Crustacea which 

 seem to adopt it as a means of escaping from the 

 attack of an enemy. On our own coasts the shore 

 8 



