Beginning with the earth-worms and their kindred, we find that at the
approach of winter they burrow deep down where the icy breath of the
frost never reaches, and there they live, during the cold season, a
life of comparative quiet. That they are exceedingly sensitive to
warmth, however, may be proven by the fact that when a warm rain comes
some night in February or March, thawing out the crust of the earth,
the next morning reveals in our dooryards the mouths of hundreds of
the pits or burrows of these primitive tillers of the soil, each
surrounded by a little pile of pellets, the castings of the active
artisans of the pits during the night before.

If we will get up before dawn on such a morning we can find the worms
crawling actively about over the surface of the ground, but when the
first signs of day appear they seek once more their protective
burrows, and only an occasional belated individual serves as a
breakfast for the early birds.

The eyes of these lowly creatures are not visible, and consist of
single special cells scattered among the epidermal cells of the skin,
and connected by means of a sensory nerve fibre with a little bunch of
nervous matter in the body. Such a simple visual apparatus serves them
only in distinguishing light from darkness, 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 autumn, crawl