Animals below the level of zoophytes and sponges are called Protozoa.
The word obviously means "First Animals," but all that we can say is
that the very simplest of them may give us some hint of the simplicity
of the original first animals. For it is quite certain that the vast
majority of the Protozoa to-day are far too complicated to be thought of
as primitive. Though most of them are microscopic, each is an animal
complete in itself, with the same fundamental bodily attributes as are
manifested in ourselves. They differ from animals of higher degree in
not being built up of the unit areas or corpuscles called cells. They
have no cells, no tissues, no organs, in the ordinary acceptation of
these words, but many of them show a great complexity of internal
structure, far exceeding that of the ordinary cells that build up the
tissues of higher animals. They are complete living creatures which have
not gone in for body-making.
In the dim and distant past there was a time when the only animals were
of the nature of Protozoa, and it is safe to say that one of the great
steps in evolution was the establishment of three great types of
Protozoa: (_a_) Some were very active, the Infusorians, like the slipper
animalcule, the night-light (Noctiluca), which makes the seas
phosphorescent at night, and the deadly Trypanosome, which causes
Sleeping Sickness. (_b_) Others were very sluggish, the parasitic
Sporozoa, like the malaria organism which the mosquito introduces into
man's body. (_c_) Others were neither very active nor very passive, the
Rhizopods, with out-flowing processes of living matter. This amoeboid
line of evolution has been very successful; it is represented by the
Rhizopods, such as Amoebæ and the chalk-forming Foraminifera and the
exquisitely beautiful flint-shelled Radiolarians of the open sea. They
have their counterparts in the amoeboid cells of most multicellular
animals, such as the phagocytes which migrate about in the body,
engulfing and digesting intruding bacteria, serving as sappers and
miners when something has to be broken down and built up again, and
performing other useful offices.
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