Animal Architecture
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most
multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry. A small
minority exhibit no symmetry (are asymmetric).
Radial Symmetry
Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface,
which contains the mouth, to the center of the opposite, or aboral, end. This type of symmetry is especially suitable
for sessile animals such as the sea anemone, floating animals such as jellyfish, and slow moving organisms such as
starfish (see special forms of radial symmetry). Animals in the phyla cnidaria and echinodermata exhibit radial
symmetry (although many sea anemones and some corals exhibit bilateral symmetry defined by a single structure,
the siphonoglyph.
Special forms of radial symmetry
Tetramerism
Many jellyfish have four canals and thus exhibit tetramerous radial
symmetry. This form of radial symmetry means it can be divided into 4
equal parts.
Pentamerism
This variant of radial symmetry (also called pentaradial and pentagonal
symmetry) arranges roughly equal parts around a central axis at
orientations of 72° apart.
Members of the phyla echinodermata (such as starfish and sea urchins)
have parts arranged around the axis of the mouth in five equal sectors.
Being bilaterian animals however, they initially develop biradially as
larvae, then gain pentaradial symmetry later on. The radiolarians
demonstrate a remarkable array of pentamerism forms. Examples include
the Pentaspheridae, the Pentinastrum group of general in the
Euchitoniidae, and Cicorrhegma (Circoporidae).
Hexamerism and octamerism
Corals and sea anemones (class Anthozoa) are divided into two groups based
on their symmetry. The most common corals in the subclass Hexacorallia
have a hexameric body plan; their polyps have sixfold internal symmetry and
the number of their tentacles is a multiple of six.
Corals belonging to the subclass Octocorallia have polyps with eight tentacles
and octameric radial symmetry.
Bilateral Symmetry
In bilateral symmetry (also called plane
symmetry), only one plane, called the sagittal plane,
will divide an organism into roughly mirror image
halves (with respect to external appearance only.
Thus there is approximate reflection symmetry.
Often the two halves can meaningfully be referred
to as the right and left halves, e.g. in the case of an
animal with a main direction of motion in the plane
of symmetry.
Most animals are bilaterally symmetric, including humans (see also facial symmetry), and belong to the group
Bilateria. The oldest known bilateral animal is the Vernanimalcula. Most bilateral animals have an identical shape
on either side, as if cut by a mirror.
Bilateral symmetry permits streamlining, favors the formation of a central nerve center, contributes to
cephalization, and promotes actively moving organisms. Bilateral symmetry is an aspect of both chordates and
vertebrates.
Asymmetric
The notable exception among animals is the phylum
Porifera (sponges) which have no symmetry.
*Directions: Using toothpicks, marshmallows and index
cards, create animal models that demonstrate the following
types of symmetry:
1. Radial symmetry (Pick 2 types to represent)
2. Bilateral symmetry (Label your drawing using the figure above)
3. Asymmetry
*After having made the models, draw each on the output side of your assignment page.