2. (Biol.) The law of likeness; similarity of structure;
regularity in form and arrangement; orderly and similar
distribution of parts, such that an animal may be divided
into parts which are structurally symmetrical.
Note: Bilateral symmetry, or two-sidedness, in vertebrates,
etc., is that in which the body can be divided into
symmetrical halves by a vertical plane passing through
the middle; radial symmetry, as in echinoderms, is that
in which the individual parts are arranged
symmetrically around a central axis; serial symmetry,
or zonal symmetry, as in earthworms, is that in which
the segments or metameres of the body are disposed in a
zonal manner one after the other in a longitudinal
axis. This last is sometimes called metamerism.
3. (Bot.)
(a) Equality in the number of parts of the successive
circles in a flower.
(b) Likeness in the form and size of floral organs of the
same kind; regularity.
{Axis of symmetry}. (Geom.) See under {Axis}.
{Respective symmetry}, that disposition of parts in which
only the opposite sides are equal to each other.