We have had occasion before
to speak of these animals, and as they are not conspicuous or
commonly-known forms of life, it may be well to say a few words
as to the structure of the living representatives of the group.
The _Foraminifera_ are all inhabitants of the sea, and are mostly
of small or even microscopic dimensions. Their bodies are composed
of an apparently structureless animal substance of an albuminous
nature ("sarcode"), of a gelatinous consistence, transparent, and
exhibiting numerous minute granules or rounded particles. The
body-substance cannot be said in itself to possess any definite
form, except in so far as it may be bounded by a shell; but it
has the power, wherever it may be exposed, of emitting long
thread-like filaments ("pseudopodia"), which interlace with one
another to form a network (fig. 25, b). These filaments can be
thrown out at will, and to considerable distances, and can be
again retracted into the soft mass of the general body-substance,
and they are the agents by which the animal obtains its food.
The soft bodies of the _Foraminifera_ are protected by a shell,
which is usually calcareous, but may be composed of sand-grains
cemented together; and it may consist of a single chamber (fig.
26, a), or of many chambers arranged in different ways (fig.
26, _b-f_). Sometimes the shell has but one large opening into
it--the mouth; and then it is from this aperture that the animal
protrudes the delicate net of filaments with which it seeks its
food.
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