ARTICULATA. 275 
Fie. 245.—Barnacles, or Pedunuculate Cirripedes (Lepas anatifera). 
gon), Cray-fish and Lobster (Astacus), and Crab (Cancer). 
Crabs differ from Lobsters chiefly in being formed for 
creeping at the bottom of the sea instead of swimming, 
and in the reduction of the abdomen or “tail” to a mere 
Fra. 246.—Acorn-shells (Balanus) on Fig. 247.—Water-fleas: 1, Cyclops communis ; 
the Shell of a Whelk (Buccinum). 2, Cypris unifasciata ; 3, Daphivia pulex. 
