not enter into our plan to speak of parasites. Yet, if among these
some turn to a host to demand of him both food and shelter, if even
they can come to be so modified and so marked by parasitism that they
can live in no other way, there are others who ask for lodging only
from an animal better protected than they are themselves. It is these
whose customs we are called upon to consider. In the interior of the
branchial chamber of many bivalvular Mollusca, and especially the
Mussel, there lives a little crustaceous commensal called the Pea-crab
(_Pinnoteres pisum_). He goes, comes, hunts, and retires at the least
alarm within his host's shell. The mussel, as the price of its
hospitality, no doubt profits by the prizes which fall to the little
crab's claws. It is even said that the crab in recognition of the
benefits bestowed by his indolent friend keeps him acquainted with
what is passing on around, and as he is much more active and alert
than his companion he sees danger much farther away, and gives notice
of it, asking for the door to be shut by lightly pinching the mussel's
gill. But this gratitude of the Crustacean towards a sympathetic
bivalve is merely a hypothesis; we do not exactly know what passes in
the intimacy of these two widely-differing natures.

For birds like the Cuckoo and the _Molothrus_ it is not possible to
plead attenuating circumstances. They occupy a place in an inhabited
house without paying any sort of rent. Every one knows the Cuckoo's
audacity. The female lays her eggs in different nests and troubles
herself no further about their fate. She seeks for her offspring a
shelter which she does not take the trouble to construct, and moreover
at the same time assures for them the cares of a stranger in place of
her own.

In North America a kind of Starling, the _Molothrus pecoris_, commonly
called the Cow-bird, acts in the same careless fashion. It lives in
the midst of herds, and owes its specific name to this custom; it
feeds on the parasites on the skin of cattle. This bird constructs no
nest. At the moment of laying the female seeks out an inhabited
dwelling, and when the owner is absent she furtively lays an egg
there. The young intruder breaks his shell after four days'
incubation, that is to say, usually much before the legitimate
children; and the parents, in order to silence the beak of the
stranger who, without shame, claims his share with loud cries, neglect
their own brood which have no