In adaptation to the great pressure the bodies of deep-sea animals are
usually very permeable, so that the water gets through and through them,
as in the case of Venus' Flower Basket, a flinty sponge which a child's
finger would shiver. But when the pressure inside is the same as that
outside nothing happens. In adaptation to the treacherous ooze, so apt
to smother, many of the active deep-sea animals have very long,
stilt-like legs, and many of the sedentary types are lifted into safety
on the end of long stalks which have their bases embedded in the mud. In
adaptation to the darkness, in which there is only luminescence that
eyes could use, there is a great development of tactility. The
interesting problem of luminescence will be discussed elsewhere.
As to the origin of the deep-sea fauna, there seems no doubt that it
has arisen by many contributions from the various shore-haunts.
Following the down-drifting food, many shore-animals have in the course
of many generations reached the world of eternal night and winter, and
become adapted to its strange conditions. For the animals of the
deep-sea are as fit, beautiful, and vigorous as those elsewhere. There
are no slums in Nature.
[Illustration: THE BITTERLING (_Rhodeus Amarus_)
A Continental fish which lays its eggs by means of a long ovipositor
inside the freshwater mussel. The eggs develop inside the mollusc's
gill-plates.]
[Illustration: _Photo: W. S. Berridge._