Body Plan and Symmetry

Body Plan

1. Cell Aggregate Body plan – The body is made up of cells (Aggregate) and each cell act almost independently. e.g. Porifera (sponges)

2. Blind Sac – The body resembles a sac and contains a single cavity with one opening that serves for ingestion and egestion both e.g., Cnidaria (Hydra), Platyhelminthes (flatworms).

3. Tube within a tube – The body is made up of two tubes, outer body wall and the inner digestive tract. e.g. Annelida

Body Symmetry

The arrangement of body parts in a geometrical pattern is called a symmetry.

Following types –

1. Spherical symmetry – Found in spherical body. All planes that pass through the center will cut it into similar halves. e.g. Some protozans like Volvox, Heliozoa, Radiolaria.

2. Radial symmetry – The body is in the form of a flat or tall cylinder. All the lines passing through longitudinal axes in a radiating manner divide the body into equal halves. e.g. Hydra.

(a) Biradial symmetry – divides the animal into two equal halves. e.g. Ctenophora and most anthozoa (e.g. Sea anemones)

(b) Pentamerous radial – divides into 5 equal parts e.g., star fish.

3. Bilateral symmetry – The body is divided into two equal parts by a single median longitudinal or sagittal plane. e.g., most higher animals.

4. Asymmetry – when an animal cannot be divided into equal parts by any plane e.g., Amoeba, most sponges.