MARINE ARTHROPODS 



261 



and a pair of compound eyes. Then as the successive moultings 

 take place, new segments and new appendages are developed, until, 



FIG. 190. FOUR STAGES IN THE DEVELOPMENT or THE COMMON 

 SHORE CRAB 



at last, the form of the adult is assumed. The accompanying 

 illustration shows four stages in the development of the Common 

 Shore Crab. 



The lowest division of the crustaceans 

 contains the Cirripedia or Curl-footed 

 crustaceans, which includes the Barnacles 

 that are so frequently seen attached to 

 the bottom of ships and of floating timber, 

 and the Acorn Barnacles, the conical shells 

 of which often completely cover large 

 masses of rock on our shores. 



For some time naturalists could not 

 agree as to the proper place of these 

 animals in the scale of life, but the matter 

 was finally settled when some minute 

 creatures only about a twelfth of an inch 

 in length, and closely resembling the early 

 stages of certain crustaceans, were seen 

 to undergo metamorphoses, and finally 

 develop into acorn barnacles. Their posi- 

 tion in the animal kingdom was thus 

 determined by their early stages ; but 



these, instead of changing into a segmented and highly organised 

 creature like the typical crustacean, lose some of their appendages, 

 cease to be free-moving animals, and attach themselves to floating 



FIG. 191. 

 THE BARNACLE 



