SEA-SHELLS ON THE SEASHORE i 47 



the shells opening by elasticity and being closed by the 

 muscle joining one to the other, at rapid intervals, flapping 

 like the wings of a butterfly. 



In the univalves the attachment of the muscle to the 

 shell gives a fixed point for all the movements of the 

 animal. The limpet has a well-marked head and neck — 

 a pair of sensitive tentacles, and a small pair of dark- 

 coloured eyes. The mouth is at the end of a sort of 

 short snout. Just within the mouth, and capable of 

 being pushed forwards to the level of the lips, is a most 

 extraordinary rasp. It consists of a long ribbon, beset 

 with fine horny teeth — very sharp and complicated in 

 pattern. The ribbon extends far back into the body, 

 and is worn away by constant use at the orifice of the 

 mouth. It grows forward, like one of our finger-nails, 

 as it wears out, and a new, unworn portion takes the 

 place of that worn away. It is constantly in use to rasp 

 and bring into the mouth the particles of the seaweed 

 on which the limpet feeds. It is easy to remove this 

 rasping ribbon with a needle or pen-knife, and examine 

 it with a microscope. Every one of the hundreds of 

 kinds of univalve molluscs has this ribbon-rasp, and its 

 teeth are of different patterns in the various kinds. It is 

 worked by very powerful little muscles, backwards and 

 forwards, and is strong enough in the whelks to bore a 

 round hole into other shells (for instance, that of the 

 oyster), when the whelk proceeds to eat the soft animal, 

 whose protecting shell has been thus penetrated. Some 

 of the large marine snails produce a poisonous secretion 

 from the mouth, which renders their attack with the 

 ribbon-rasp all the more deadly to other marine creatures. 

 The cuttle-fishes and octopods, which are molluscs too, 

 possess, like the univalve limpets, snails, and whelks, this 

 terrible ribbon-rasp in the mouth. It is an indication of 



