MOLLUSCA, THE SHELL-FISH 105 



these also may be found displaying the deadly 

 little round hole we have described. 



Let us contrast with the Murex one of the 

 shells which are " holostomatous," i.e. possessing 

 an unindented shell-mouth that is to say, one 

 without a "siphon." The common edible peri- 

 winkle, Littorina littorea,, may be taken as an 

 example. No shell is more familiar; even the 

 town-dweller, who has never found it on the sea- 

 shore, has seen it often on stalls in the slums. 

 The mouth of the shell is quite round and unin- 

 dented, and in this case the character holds good 

 as the mark of a vegetable-feeder a non-preda- 

 tory sea-snail. It i? hardly necessary to remind 

 the reader that its name (the shore-shell) is given 

 it because it lives where the tide leaves the rocks 

 exposed during part of the day. Another common 

 species of Littorina, which frequently lives a little 

 lower down, where the large sea-weeds grow, has 

 been described in Chapter II. ; and another, L. 

 rudis, lives a little higher up, so that it spends 

 most of its time in a dry state, and is fast on its 

 way to become a land-shell. At most of the 

 familiar English seaside resorts one may see 

 dozens of it baking in a hot July sun on rocks 

 where only the highest tides can reach them : and 

 yet under these conditions they continue to live 

 and flourish. The periwinkles are remarkable for 

 the great length of the tooth-ribbon, in compari- 

 son with the size of the animal. The number of 

 separate teeth upon it has been estimated at 3500. 



A familiar feature of the common periwinkle 

 is the lid or stopper (Operculum), with which 

 the animal can close the mouth of the shell. This 

 is developed and carried by the outside of the 

 animal's foot. In the periwinkle and other Eng- 



