SYMMETRY
The symmetry refers to the arrangement of parts on the
opposite sides of the body of a three dimensional animal.
On the basis of symmetry, animals can be of following types
1. Asymmetrical
Animals in which. any plane passes through the centre does not
divide them into equal halves such animals are called
asymmetrical. e.g., sponges.
2. Symmetrical
The body of some animals can be divided into two similar equal
halves by one or more planes. Such animals are called
symmetrical.
The symmetry can be further divided as
i. Radial Symmetry
hen any plane passing through the central axis of the body
divides the organism into two identical halves, it is called radial
symmetry. e.g.. coelenterates, ctenophores and echinoderms.
u Tien the body can be divided into two similar halve
by one or two vertical planes only, the radial symmetry
is called biradial symmetry. e.g.. sea anemones.
Adult sponges are asvmmetrical, however they star
their life form as a radial symmetrical larvae.
ii. Bilateral Symmetry
In some animals. body can be divided into identical left
and right halves in only one plane. This is called
bilateral symmetry, e.g., annelids, arthropods, etc.