With the exception of cnidarian s, such as jellyfish, and echinoderms, most animals are bilaterally symmetrical. This means that their body can be divided into similar halves along a line drawn from head to tail. This is the first part of a mobile animal to reach new stimuli in its environment, and hence contains a vast array of sensory organs. The high activity levels of most animals, and their large multicellular bodies, necessitated the evolution of efficient circulatory systems to move oxygen and food to the tissues, and remove waste products. Larger bodies also need good excretory and nervous systems.
Animals comprise one of the kingdom s of living organisms.
The link between animals and other organisms is revealed in the basic chemical reactions which sustain life. Processes such as respiration , and the enzymes used in many similar biochemical pathways, are common to all kingdoms. Oxford University Press 3. All rights reserved.