ii4 THE LIFE OF CRUSTACEA 



collector will often find, on turning over a lar 

 stone, one or more specimens of the little Porcelain 

 Crabs (Porcellanaplatycheles, or P. longicornis Fig. 41) 

 clinging to its under-side. If these Crabs be seized 

 by one of the large claws, they frequently leave the 

 claw in the captor's hand and scuttle off without it ; 

 and it cannot be doubted that, as in the case of 

 lizards and other animals which have a similar power 

 of self-mutilation, this habit often enables them to 

 escape from their natural enemies. 



Although the Crustacea as a whole are pre- 

 dominantly active animals, many examples have 

 already been mentioned of species which are more 

 or less sluggish and sedentary in their habits. The 

 extreme degree of passivity is reached by the 

 Barnacles (Cirripedia), which differ from all other 

 Crustacea (except some parasites) in being fixed to 

 one spot, and quite without the power of locomotion 

 in the adult state. Most of the Barnacles met with 

 on the shore or in shallow water belong to the 

 division of the Sessile Barnacles or Acorn-shells 

 (Operculata). Every visitor to the seashore has 

 noticed the little conical shells which cover exposed 

 rocks as if with a coat of rough-cast. On the 

 British coasts the commonest species is Balanus 

 balanoides (Plate III.), though other species closely 

 resembling it are also common. They are to be 

 found almost up to high-water mark in situations 

 where they are left uncovered for many hours every 



