In animals like sea-anemones and jellyfishes the general symmetry of the
body is radial; that is to say, there is no right or left, and the body
might be halved along many planes. It is a kind of symmetry well suited
for sedentary or for drifting life. But worms began the profitable habit
of moving with one end of the body always in front, and from worms to
man the great majority of animals have bilateral symmetry. They have a
right and a left side, and there is only one cut that halves the body.
This kind of symmetry is suited for a more strenuous life than radial
animals show; it is suited for pursuing food, for avoiding enemies, for
chasing mates. And _with the establishment of bilateral symmetry must be
associated the establishment of head-brains_, the beginning of which is
to be found in some simple worm-types.
Among the other great acquisitions gradually evolved we may notice: a
well-developed head with sense-organs, the establishment of large
internal surfaces such as the digestive and absorptive wall of the
food-canal, the origin of quickly contracting striped muscle and of
muscular appendages, the formation of blood as a distributing medium
throughout the body, from which all the parts take what they need and to
which they also contribute.
Another very important acquisition, almost confined (so far as is known)
to backboned animals, was the evolution of what are called glands of
internal secretion, such as the thyroid and the supra-renal. These
manufacture subtle chemical substances which are distributed by the
blood throughout the body, and have a manifold influence in regulating
and harmonising the vital processes. Some of these chemical messengers
are called hormones, which stimulate organs and tissues to greater
activity; others are called chalones, which put on a brake. Some
regulate growth and others rapidly alter the pressure and composition
of the blood. Some of them call into active development certain parts of
the body which have been, as it were, waiting for an appropriate
trigger-pulling. Thus, at the proper time, the milk-glands of a
mammalian mother are awakened from their dormancy. This very interesting
outcome of evolution will be dealt with in another portion of this work.