PLANTS AND ANIMALS IN WINTER 317 



of a sensory nerve fiber with a little bunch of nervous 

 matter in the body. Such a simple visual apparatus 

 serves them only in distinguishing light from dark- 

 ness, but this to them is most important knowledge, 

 as it enables them to avoid the surface of the earth by 

 day, when their worst enemies, the birds, are in active 

 search for them. 



The fresh-water mussels and snails and the crayfish 



burrow deep into the mud and silt at the bottom of 



ponds and streams where they lie motionless during 



the winter. The land snails, in late 



Mussels and autumn craw ] beneath logs, and, bur- 

 Snails in Winter. . . . J? ,, ' 



rowing deep into the soft mold, they 



withdraw far into their shells. Then each one forms 

 with a mucous secretion two thin transparent mem- 

 branes, one across the opening of the shell and one 

 a little farther within, thus making the interior of 

 the shell perfectly air-tight. There for five or six 

 months he sleeps, free from the pangs of hunger and 

 the blasts of winter, and when the balmy breezes of 

 spring blow up from the south he breaks down and 

 devours the protecting membrane and goes forth with 

 his home on his back to seek fresh leaves for food 

 and to find for himself a mate. 



Next in the scale come the insects, which comprise 

 four-fifths of all existing animals, and each one of the 

 mighty horde seen in summer has passed the winter 

 in some form. One must look for them in strange 

 places and under many disguises; for they can not 

 migrate, as do the majority of the birds, nor can they 

 live an active life while the source of their food sup- 

 ply, the plants, are inactive. 



