Concept preview

What you will be able to do

1. 01Describe the three types of body symmetry and give an example animal for each.
2. 02Explain cephalization, germ layers and the body cavity (coelom).
3. 03Distinguish protostome from deuterostome development at an introductory level.
4. 04Distinguish an adaptation from acclimation and classify the types of adaptation.

One of the most powerful ways to make sense of animal diversity is to look at body symmetry — the way an animal's parts are arranged around its centre. The simplest animals, such as most sponges, have no symmetry at all; they are asymmetrical, with an irregular shape that cannot be divided into matching halves. A great advance came with radial symmetry, seen in cnidarians such as the sea anemone and the jellyfish: their body parts are arranged like the spokes of a wheel around a central axis, so that any cut through the centre produces mirror-image halves. Radial symmetry suits animals that are sessile or slow-drifting, because it lets them sense and capture food arriving from any direction equally well. Most animals, however, show bilateral symmetry — a distinct left and right side that mirror each other, together with a front and back and a top and bottom.