CRUSTACEA AND MOLLUSCS 239 



ruminants. Natural selection can certainly cause the 

 adult animal to adopt a diet different from that it was 

 accustomed to in its youth. 



We have a good illustration of such a development 

 of diet in the second great group of animals with which 

 we have to deal in this chapter, the "molluscs." The 

 chief representatives of this stem amongst us are mussels 

 and snails. Of the latter, the land-covered snails eat 

 fungi or mould, and the road-snails fungi. But a few 

 species in each family have taken to a flesh diet. They 

 can, of course, only catch slow animals such as earth- 

 worms or other snails. And it is only certain species 

 of snails that they eat, while avoiding the rest, possibly 

 because they are protected by a strong slimy secretion. 



The mucus is a general characteristic of the snails. 

 Our water-snails use it for creeping on the surface, 

 many experts believe, and as a fact this seems to be 

 the simplest explanation of the mysterious progress of 

 the pond-snail on the surface of the water, with its 

 body bent downwards. The snails which seem to 

 have the flat part hanging in the air, and to creep on 

 this, give out a long slimy thread and glide along this. 



The water-snails have a peculiar organ that enables 

 them to rise or sink without effort, as the fishes do. 

 This is their breathing organ, a cavity in the body that 

 opens externally by an orifice. The animal sinks by 

 compressing the cavity, and rises again by dilating it. 



The respiratory cavity is the lung of our molluscs, and 

 is found both in the land and water-snails, as the latter 

 are land-animals that have passed into the water, and 

 must come to the surface to breathe and close the cavity 



