move in all
directions without reference to any definite axis, or they move in one
main direction.

The Amorphozoa or shapeless animals include many of the Protozoa and
sponges; they have no typical form, and most of them are sessile. The
Actinozoa include such animals as the Coelentera, which are fixed, and
the Echinoderms, which have a central point and move indifferently along
any radial axis; their form differs from the strobiloid mainly in having
radiate rather than spiral symmetry. The Hemisphenozoa, or bilaterally
symmetrical animals, include all those that habitually move forward;
they have a front end and a hind end, a dorsal surface and a ventral,
and the mouth, sense-organs and "brain" are concentrated in the front
end to form a head--all in direct adaptation to this forward movement;
they make up the vast majority of animals.

The fundamental forms of living things are, however, merely so many
themes on which a multitude of further variations are woven, through the
action of the laws which rule the detail of organic diversities. These
further laws may be set down under four main heads. Under the first
comes the law of the existence of certain fundamentally distinct
structural types, which are distinguished from one another by their
ground-form, by the number of organ-systems, and by the number of
homotypic organs they possess, but principally by the relative position
of the organs to one another (principle of connections). The form and
connections of the nervous system are of particular importance in
distinguishing the types (_cf._ Cuvier). The second factor in the
diversity of organic form is the action of certain laws of progressive
development[313] (_Entwickelungsgesetze_), which bear the same relation to
the development of the animal kingdom as the laws of individual
development bear to the development of the embryo, for organs appear in
the different animal series in much the same order and manner as they
develop in the individual. These laws are (1) progressive
differentiation of functions and organs; (2) numerical reduction of
serially repeated parts; (3) concentration of functions and their organs
in particular parts of the body; (4) centralisation of organ-systems and
parts of such, so that they come to depend upon one central organ; (5)
internalisation of the "noblest" organs, unless these are necessarily
external, and (6) increase in size of the whole or of parts. Of these
the law of differenti