ARTHROPODA 



79 



Among the most curiously modified forms of 

 the Crustacea are the Barnacles or Cirripedia. 

 These creatures, like the sponges, have a free- 

 swimming larvae, which eventually fixes itself by 

 its anterior end, so that the adult animal passes 

 its existence upside 

 down. The young is 

 an ordinary little 

 creature with jointed 

 legs, but the adult 

 protects itself by a 

 strange armour of 

 shell. An intermedi- 

 ate stage exists in 

 which the creature 

 eats no food ; it has 

 therefore been com- 

 pared with the chrysa- 

 lis of insects. At the 

 top of the adult shell 

 two little valves open 

 and shut, allowing 

 the legs to dart out 

 and seize upon prey. 

 These legs, gathered 

 into a bunch, and ex- 

 tended and retracted 

 together, remind one 

 of the fingers of a hand 

 opening and closing. 

 They are clothed with 



FIG. 14. Shell of the Bell Barna- 

 cle, Balanus tintinnabulum, 

 one-halt the natural size. The 

 figure shows several successive 

 generations, perched one upon 

 another. 



a fringe of " cirrhi " 

 or small processes; hence the name of the group. 

 The Common Barnacle of our own shores, some- 

 times called the Acorn-Shell, is found on shells and 

 stones, and often on those that are left uncovered 

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