432 TEXT-BOOK OF ZOOLOGY 



extent neutralized by the possession of the hard shell. (Why cannot an 

 active animal constantly carry its "hiding-place " about with it?) 



3. The organB required for the discovery of food and of foreign objects 

 generally are naturally to be found upon that part of the body which 

 during motion is " to the front," viz., the head (see, however, Lamelli- 

 branchiata). These organs consist of two pairs of feelers (" horns "), of 

 which the upper and longer pair carries the eyes, visible in the form of 

 black dots. At the slightest touch these hollow tentacles are retracted, 

 being introverted like the fingers of a glove ; and it is only gradually and 

 after carefully feeling about that they are once more protruded. The 

 terminal knobs of the long feelers also serve as organs of smell. 



4. The snail must also possess organs which will enable it to cut off 

 very fine particles from plants (compare also Part I., p. 102). These 

 organs and the manner in which they are employed may be best observed 

 in a water-snail as it creeps along the glass side of an aquarium, feeding 

 upon the green algse with which this is covered. The lips of the mouth, 

 previously closed, separate, and reveal a small brown plate, the so-called 

 jaw. In addition to this, a tongue-like structure, covered with a mem- 

 brane (composed of conchiolin), is protruded from the buccal cavity. 

 When viewed under the microscope, this organ is seen to be covered 

 with thousands of very fine backwardly-pointed teeth, so that it bears 

 a remarkable resemblance to an elegant rasp or minute file, and is 

 known as the radula. Every time that the snail presses the radula 

 against the glass pane, moving it at the same time from below upwards, 

 it scrapes off with it a small strip of the algous covering. When the 

 food is of a harder nature (leaves and stalks), the jaw also is brought 

 into action, cutting off such particles of the plant as are pressed 

 against it by the radula. The mouth parts of the vineyard snail are 

 constructed essentially on the same plan, and it feeds in exactly the 

 same manner ; but owing to its diet consisting exclusively of leaves, the 

 mouth parts can only rarely be seen in action. It is, however, easy to 

 discover the traces left by the radula (on the surfaces of leaves, especially 

 distinct in fungi) and by the jaws (at the bitten edges). Indeed, by 

 placing a number of these snails upon cabbage lettuces, the operations of 

 the mouth parts may actually be distinctly heard. The jaws and radula 

 may be easily obtained by boiling the head of a snail in a solution of 

 potash. The intestine, which is continuous with the mouth, traverses 

 the coils of the body, and terminates on the left side, close to the 

 respiratory aperture. 



5. In autumn the snail, by the aid of its foot, burrows in loose 

 leaf-and-moss-covered soil, closes the aperture of the shell with a hard, 

 calcareous lid (epiphragm), and enters upon a stage of hibernation. 



