The concept of symmerty is fundamental in understanding the organisation of an animal. During the course of evolution, animals acquired several body forms all of which fall into few types of symmetry.
The symmetry of an animal generally fits its life style.
E.g:- Radial symmerty is an advantage to sessile or planktonic or slow-moving Orgnisms. In Radial symmetry the sensory and feeding structures are uniformly distributed around the body. They can interact with their environment equally in all the directions they can capture prey approching from any directions
Bilateral animals generally move actively from place to place. Their central nervous system eanbles them to co-ordinate complex movements involved in crawling, burrowing, flying or swimming.
Symmetry in animals is balanced distribution of paired body parts
The animals, which can not be cut into two equal parts in any plane through the centre of the body are called a Symmetrical.
In a Symmetry animals –
• The body lack a definate form
• The body can not be divided into mirror halves by any plane.
• They do not develop complex sensory and locomotor functions.
E.g: Ameoba, most of the Sponges, adult Gastropodes.
The regular arrangement of body parts in a Geometrical design, relative to the axis of the body is called symmetry.
The animals which can be cut into two equal parts, or antimeres in one or more planes passing through the principal axis of the body are called ‘symmetrical animals’.
In a Symmetrical animal –
• Paired body parts are arranged on either side of the plane passing through the principal axis, at equidistant from the plane.
• The unpaired body parts are located mostly on the plane, passing through the principal axis.
The Symmetry in animals is of different kinds. They are-
1. Spherical Symmetry.
2. Radial Symmetry.
3. Biradial Symmetry.
4. Bilateral Symmetry.
1. Spherical Symmetry : ( Homaxial Apolar Symmetry)
• The body has the form of a sphere.
• Body parts arranged concentrically around a central point.
• All axes passing through the central point are apolar.
• Any one of the infinate number of planes that pass through the central point divides the body into antimeres.
• Spherical forms are best suited for floating and rolling.
• This type f Symmetry is rare.
• It is found in some Protozoans only.
II. Radial Symmetry : ( Maximal Heteropolar Systems)
When any Plane passing through the oro-aboral axis of the body divides an Organism into two antimeres (Identical Polars), it is called Radial Symmetry.
• The body is cylindrical (e.g:Hydra) or
Vase-like (e.g:Sponges) or
Umbrella-Shaped (e.g: Jelly fish)
• These animals are either sessile (sponge) or Sluggish or planktonic forms.
• The principal axis is oral-aboral axis. It is heteropolar (two ends are unlike)
• Anterior, Posterior ends and dorsal, ventral, lateral surfaces are not differentiated.
• Various body parts are arranged aroung and along the oral-aboral axis at equidistance.
E.g: Some Sponges,
Most of the enidarians- ( Hydra, Jelly fishes)
Adult forms of echinoderms ( Pentamerous radial)
III Biradial symmetry.
1.Pired arrangement at some structures in radially symmetrical animals results in biradial symmetry.
2. There are two planes of symmetry, one passing along the oral- aboral axis and the long axis of mouth,
The other passing along the oral- aboral axis and the short axis of mouth.
3. The antimeres on either side of one plane are different from the antimeres on either side of the other plane.
4. The principal axis is the oro-aboral axis, it I heteropdar. Eg: Ctenophores and mot Anthozoans(sea anemones)
• In Ctonophores most of the body parts are radially arranged but the tentacles are paired. Such paired arrangement of some structures in radially symmetrical animals results in biradial symmetry.
• In Seaanemone, the biradial symmetry seems to have been derived from the radial type primarily by the elongation of the mouth and assosiated parts.
When only one plane [ Median sagittal plane] that passes through the central axis [ anterior – posterior axis ] divides an organism into two identical parts , it is called bilateral symmetry.
• Bilateral symmetry is the pricipal type of symmetry in the triploblastic animals.
• Bilaterally symmetrical animals are more efficient than the other animal in seeking food, locating mates and escaping from predators.
• Cephalisation is the most advanced feature in bilaterally symmetrical animals.
• As a result of cephalization, these animals can sense the new environment and respond more efficiently and quickly.
• The pricipal axis is the anterior – posterior axis. It is heteropolar, with differentiated anterior and posterior ends.
• Sagittal axis is heteroplar, with differentiated dorsal and ventral surfaces.
• The transverse axis is apolar.
Eg: Triploblastic animals.