eneath several centimetres of sand, which supports it like a
vault. It forms a solid roof, beneath which the eggs undergo their
evolution. Sometimes the male remains by the little chamber to watch
over their fate. It is possible to distinguish the two holes of
entrance and exit which mark his habitual passage.

[Illustration: FIG. 19.]

The Hermit-crab perhaps knows best how to take advantage of old
clothes. (Fig. 19.) He collects shells of Gasteropods, abandoned
flotsam, the first inhabitant of which has died. The Hermit-crab
(_Pagurus Bernhardus_) is a Decapod Crustacean--that is to say, he
resembles a very small Crab. But his inveterate habit during so many
generations of sheltering his abdomen in a shell prevents this part
from being encrusted with lime and becoming hard. The legs and the
head remain in the ordinary condition outside the house, and the
animal moves bearing it everywhere with him; on the least warning he
retires into it entirely. But the Crustacean grows. When young he had
chosen a small shell. A Mollusc, in growing, makes his house grow with
him. The Hermit-crab cannot do this, and when his dwelling has become
too narrow he abandons it for one that is more comfortable. At first
enclosed in the remains of a _Trochus_, he changes into that of a
_Purpura_; a little later he seeks asylum in a Whelk. Beside the
shelter which these shells assure to the Crustacean, they serve to
mask his ferocity, and the prey which approaches confidently what it
takes to be an inoffensive Mollusc, becomes his victim.

The Great Horned Owl likewise does not construct a nest; but takes
possession of the dwellings abandoned by others. These birds utilise
for laying their eggs sometimes the nest of a Crow or a Dove,
sometimes the lair which a Squirrel had considered too dilapidated.
The female, without troubling about the bad state of these ruins, or
taking pains to repair them, lays her eggs here and sits on them.

_Classification of artificial shelters._--It is time to turn to
animals who have more regard for comfort, and who erect dwellings for
themselves or their offspring. These dwellings may be divided into
three groups: (1) Those which are hollowed in earth or in wood; (2)
those which in the simplest form result from the division of material
of any kind; then, as a complication, of materials bound together;
then, as a last refinement, of delicate materials, such as blades of
grass or threads of wool woven together;