plane passing through the centre of the body does not divide them into two equal halves (Spong- es). An asymmetrical body plan is also seen in adult gastropods (snails). 6SRQJLOOD YDX[LD ([WLQFW /HXFRVROHQLD Figure . Asymmetry in sponges Symmetrical animals have paired body parts that are arranged on either side of a plane passing through the central axis.
When any plane passing through the central axis of the body divides an organism into two identical parts, it is called radial symmetry . Such radially symmetrical animals have a top and bottom side but no dorsal (back) and ventral (abdomen) side, no right and left side. They have a body plan in which the body parts are organised in a circle around an axis. It is the principal symmetry in diploblastic animals.
Cnidarians such as sea anemone and corals (Figure . ) are radially symmetrical. However, triploblastic animals like echinoderms (e.g., starfish) have five planes of symmetry and show Pentamerous radial symmetry . Animals which possess two pairs of symmetrical sides are said to be Figure .
Germinal layers . . Patterns of Symmetry Symmetry is the body arrangement in which parts that lie on opposite side of an XI Std Biology-Zoology Chapter- biradially symmetrical (Figure . ).
Biradial symmetry is a combination of radial and bilateral symmetry as seen in ctenophores. There are only two planes of symmetry, one through the longitudinal and sagittal axis and the other through the longitudinal and transverse axis. (e.g., Comb jellyfish – Pleurobrachia ) Animals which have two similar halves on either side of the central plane show bilateral symmetry (Figure . ).
It is an advantageous type of symmetry in triploblastic animals, which helps in seeking food, locating mates and escaping Figure . Biradial symmetry in comb jelly Figure . Bilateral symmetry in Insects from predators more efficiently. Animals that have dorsal and ventral sides, anterior and posterior ends, right and left sides are bilaterally symmetrical and exhibit cephalisation, in which the sensory and brain structures are concentrated at the anterior end of the animal.
. . Coelom The presence of body cavity