limbs. The sponge
continues to prosper and to spread over the Crustacean who has adopted
it. (Fig. 23.) The two beings do not seem to be definitely fixed to
each other; the contact of a sudden wave will separate them. When the
divorce is effected, the _Dromia_ immediately throws itself on its
cherished covering and replaces it. M. Kuenckel d'Herculais tells of
one of these curious crustaceans which delighted the workers in the
laboratory of Concarneau. The need for covering themselves experienced
by these Crabs is so strong that in aquariums when their sponge is
taken away they will apply to the back a fragment of wrack or of
anything which comes to hand. A little white cloak with the arms of
Brittany was manufactured for one of these captives, and it was very
amusing to see him put on his overcoat when he had nothing else
wherewith to cover himself.[88]

[88] Brehm, edition Francaise, _Crustaces_, p. 738.

In these two cases which I have brought forward to exhibit the
rudiments of this industry, the animals' reflection and will play but
a small part; even in the _Dromia_ custom is so inveterate in the race
that it has reacted on the animal's organisation, and its four
posterior legs are profoundly modified for the purpose of firmly
holding the sheltering sponge; they no longer serve for swimming or
walking. The animals of which I have now to speak possess more
initiative; although all do not act with the same success, or show
themselves equally skilful. Let us turn first to the least
experienced.

An Australian bird, the _Catheturus Lathami_, as described by Gould,
is still in the rudiments, and limits itself to preparing an enormous
pile of leaves. It begins its work some weeks before laying its eggs;
with its claws it pushes behind it all the dead leaves which fall on
the earth and brings them into a heap. The bird throws new material on
the summit until the hole is of suitable height. This detritus
ferments when left to itself, and a gentle heat is developed in the
centre of the edifice. The _Catheturus_ returns to lay near this
coarse shelter; it then takes each egg and buries it in the heap, the
larger end uppermost. It places a new layer above, and quits its
labour for good. Incubation takes place favoured by the uniform heat
of this decomposing mass, hatching is produced, and the young emerge
from their primitive nest.

[Illustration: FIG. 24.]

Birds are not alone in constructing temporary dwellings i