THE ROCK-BARNACLE 65 



problem still more, and it has in most cases no visible 

 sutures, but sutures there are, however concealed. Darwin 

 found that many rock- barnacle shells fall to pieces when 

 boiled in caustic potash, separating along the sunk lines 

 which are externally visible. Each of the strong ridges 

 grows by the continual addition of fine laminae to its base. 

 Since the shell remains fixed all the time, this mode of 

 growth would probably be impossible if the shell were a 



FIG. 14. Rock-barnacle removed from its shell, in side-view. 

 The six legs with their fringed joints, the muscles for attachment 

 to the shell, &c., are shown. 



dead, structureless deposit. It is not really such, for its 

 substance is traversed by innumerable cavities, which 

 lodge living tissue, and its growth is similar to that of 

 tooth or bone, growth and replacement going on together. 

 Another kind of barnacle is often found attached to 

 objects which have long floated in the sea, such as the 

 timbers of a wreck, or corks torn off from a fisherman's 

 net. This second kind of barnacle has a stalk, and at the 

 end of the stalk is what we should at first sight call a 

 head (though it is nothing of the sort), which is defended 

 by five calcareous plates. If a stalked barnacle is broken 



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