90 THE SEA-SHORE 



wriggle its way along with a curious "screwing" 

 movement, just as though it were trying to bore 

 its way into the sand. 



PLATE XXX 



ACORN SHELLS (i) 



If you examine the rocks which are left dry 

 when the tide goes out, you will often find that 

 they are covered almost all over with small shells 

 which look rather like those of tiny limpets. 

 Only at the top of each shell there is a little 

 hole, from the margin of which a number of ridges 

 run down to the bottom. And these ridges are 

 so sharp, that if you happen to slip when you 

 are wandering about among the pools, and catch 

 at a rock to save yourself, they will cut your 

 fingers almost as if they were knives. 



These creatures are generally known as "Acorn 

 Shells," and I dare say that you might think 

 that they must be very closely related to the 

 limpets. But in reality they are much more 

 closely related to the shrimps and sandhoppers, 

 though they look so very unlike them, and lead 

 such different lives. For while shrimps and sand- 

 hoppers are always swimming or skipping about, 

 the little animals which live inside these acorn 

 shells never move at all after they are a few 



