LUNG-BREATHING GASTROPODS 431 



C. The Snail a Vegetable-feeder. 



1. The snail creeps along in search of its food, which consists of 

 leaves. Locomotion is accomplished by means of the muscular foot with 

 its broad sole. If a snail is allowed to crawl up a glass pane, it will be 

 seen that the sole of the foot is not lifted off from the surface of support 

 (like the foot of a vertebrate, for instance), but glides slowly along. Only 

 a few wavelike movements may be seen passing over it from behind 

 forwards, something like cloud shadows over the earth. We further see 

 that with each commencing wave the posterior end of the sole is drawn 

 somewhat forwards, and with each concluding one the anterior end is 

 extended somewhat forwards, while the rapidly-succeeding waves slowly 

 impel the animal forwards. (How this peculiar motion is effected is not 

 yet accurately known.) 



Whether a snail be placed on loose or hard soil, on glass, wood, or 

 any other material, it always creeps along at the same speed. This is 

 undoubtedly a remarkable phenomenon, for we all know how the friction 

 between the soles of our feet and the supporting surface varies according 

 as we pass over different kinds of soil (e.g., a rocky or sandy soil), 

 and how a loose soil impedes our progress. This drawback the snail 

 overcomes by means of the band of slime, or mucus, which the animal, as 

 it were, spreads out upon its path. As we must all have noticed, this 

 tough mucus adheres firmly to the surface of support, but separates with 

 ease from the sole of the foot by which it is secreted. Hence, in the 

 case of the snail we are concerned, not with friction between the sole and 

 the supporting surface, but with that existing between the sole and the 

 mucus. This latter friction, however, being always the same, the 

 animal's speed of progress is consequently likewise invariable. 



By means of this remarkable organ of locomotion the snail is able to 

 ascend even vertical objects (trunks of trees, rocks, etc.), and to creep 

 along the under side of horizontal surfaces (leaves). The sole in this 

 operation acts something like a sucker (see cuttle-fish), or the adhesive 

 pads which we have noticed in the foot of the fly. The viscidity of the 

 mucus must also be considered in this connection. 



2. Compared with mammals and arthropods, which are possessed of 

 articulated limbs, the snail is a proverbially slow-moving creature (we 

 talk of " snail's pace," etc.). Nor is there any necessity for rapid move- 

 ment (any more than in the case of a caterpillar) , the animal being a 

 vegetable-feeder, able to obtain its food without great exertion. (Hence 

 predaceous gasteropods are only able to capture slowly-moving animals.) 

 The sluggishness of the snail, however, is attended by many dangers. 

 (Enemies: Starling, thrush, shrewmice, etc.) It is, however, to some 



