THE ROCK-BARNACLE 



63 



from one thing to another. We will not load our memories 

 with names, nor with brief descriptions hardly more pro- 

 fitable than mere names ; let us pitch upon some one 

 thing and consider it attentively. If local abundance may 

 decide our choice, we can hardly find anything better than 

 the little barnacles which stud the rocks with their dirty- 

 white shells in such numbers that they often form a grey 

 or yellowish band which can be seen a mile or two away. 



The common rock-barnacle of the north of England is 

 a rather small species, 

 less than half an inch 

 in diameter at the base, 

 which is the widest 

 part. On our southern 

 coasts this species is 

 replaced by others of 

 the same general ap- 

 pearance, but differing 

 slightly in details. 

 There are much larger 

 species than these, and 

 many of us have seen 



a pink or purple barnacle as big as a walnut, which is 

 often found in large quantities on the bottoms of ships 

 arriving from tropical ports. Rock-barnacles commonly 

 adhere so firmly to their support that they can hardly be 

 removed without fracture. They are glued fast by a 

 calcareous substance almost as hard as the shell itself. 

 In our northern rock-barnacle the sides of the conical 

 shell are divided into six ridges by narrow sunk spaces, 

 and every ridge is marked by several radiating ribs, but 

 the surface-pattern is often obscured by wear or corrosion. 

 At the summit of the cone is a hole closed by two pairs 

 of close-fitting valves, which can be raised or lowered a 

 little, or parted to allow of the entrance of water and the 

 protrusion of parts of the body. If live rock-barnacles are 

 placed in sea-water and watched with a lens, the valves 



FIG. 13. Group of rock-barnacles, 

 slightly enlarged. 



