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 a common parentage or ancestral relationship in the forms which possess it. The cockle (Fig. 19, d), to which we now turn, has not got a ribbon-rasp, nor anything of the kind. It has a mouth with four flapper-like lips, but no projecting head, no eyes, no biting mechanism, nor have any of the bivalves, excepting a few which like the scallop have a series of eyes on the edge of the soft mantle or flap which lines the shell. This constitutes a greater difference between bivalves and the univalves than does the shape of the shell. They are a very quiescent, peaceful lot, feeding on microscopic floating plants (diatoms and such), which are drawn to the mouth by currents of water set going by millions of vibrating hairs arranged on four soft plates hanging under the protecting arch of the shell, and called in the oyster--in which bivalve most people know them--the "beard."