go THE LIFE OF CRUSTACEA 



a poison that paralyzes its victims. Some years ago 

 a "plague" of Octopus very seriously affected the 

 Lobster fishery in the English Channel. To escape 

 from enemies such as these, the Lobsters and many 

 Crabs have the habit of lurking in crevices of the 

 rocks, while in case of sudden alarm the Lobster 

 may escape from danger by swimming, or rather 

 darting, with great swiftness, tail foremost, through 

 the water by powerful strokes of the abdomen and 

 tail-fan. In the more lightly armed Prawns and 

 other Crustacea of the tribe Natantia, which are 

 characteristically swimmers, the power of rapid 

 motion is probably the chief means of protection 

 against enemies. There is reason to believe that 

 the Lobsters have been derived from prawn-like 

 swimming forms which have sacrificed some of their 

 agility in developing their heavy armour -plating, 

 retaining, however, the power of sudden and rapid 

 motion in emergency. This power, again, has been 

 lost by the typical Crabs (Brachyura), in which the 

 abdomen is reduced in size and without a tail-fan, 

 so as to be useless for swimming. While most of 

 the Crabs, however, are somewhat slow of move- 

 ment, trusting to their armour and their powerful 

 pincers for defence, the Swimming Crabs (Portunidse 

 — Plate XIII.) have reacquired the power of swimming 

 by means of the paddle-shaped legs of the last pair. 

 Some of the tropical species of Portunidae are prob- 

 ably the most expert swimmers among the Crustacea, 



