THE FOOD OF SNAILS 73 



in natural history. So intimately are they dependent 

 on one another that some plants, such as certain figs 

 and the Yucca palm, would die out were it not for a 

 small wasp in the one case and a minute moth in the 

 other, that serve, and alone serve, to ensure fertilisation. 

 Another method of obtaining vegetable food is 

 that practised by snails and slugs. These animals 

 browse on aquatic and terrestrial plants by rasping 

 away the surface of the plant between their prickly 

 tongue and horny upper jaw. This method is, on a 

 small scale, the one adopted by ruminants, such as cows 

 and sheep, which grip their food between the tongue 

 and horny pad of the upper jaw, and in both cases the 

 lips are used to bring the food to the teeth. But in 

 the snail the tongue is covered with a ribbon bearing 

 horny teeth, which as fast as they wear away are 

 replaced by new ones, and the action of these teeth is 

 not unlike that of the cutter in a lawn-mower, while 

 the fixed upper jaw of the snail corresponds to the 

 bar against which the cutter clips off the grass. By the 

 aid of this rasping tongue snails and slugs are able 

 to obtain food in almost all circumstances. The 

 covering of slimy alga on the seashore, the green 

 scum that appears on pools and roadsides, the liver- 

 worts, mosses, fungi, lichens, and most of the higher 

 plants, provide an endless store of provender. So 

 great is the injury done by snails, without any counter- 

 balancing advantage, such as many insects confer, 

 that plants have sought to protect themselves against 

 these attacks by various methods. So general is this 



