Most animals are ‘bilateral’, in other words they have a head and tail end, and a left and right. Some have ‘radial symmetry’ (roughly circular with no front or back), and yet others show no symmetry at all.
Bilateral Symmetry
- Most animals have a definite head end, and their bodies are typically long and thin. Legs or fins are in pairs, with the left being mirror images of the right. This arrangement allows the animal to be streamlined, as in fish, and to develop a clear head that moves through the environment first and houses important sense organs. Bilateral animals are very well adapted for moving around – bodies can be wiggled (snakes and fish), or paired limbs can be used for waking. Having the majority of sense organs in one place near the mouth allows efficient hunting for mobile prey.
- Echinoderms are unusual in that they have bilateral larvae, but adults that look more ‘radial’. These adults usually show a special kind of symmetry called ‘pentamerous’ (five-fold symmetry), with 'arms' in sets of five around the mouth. The larvae all swim, but the adults either move slowly or are permanently attached to the substrate.
Radially symmetrical animals are very well suited to filter feeding, where there is no real need to move about – food is found in the water surrounding the animal.
- Jellyfish are usually circular in shape, but a close look often reveals a four-fold symmetry with organs found in fours.
- Corals and sea anemones can have either eight-fold or six-fold symmetry superimposed, with tentacles found in multiples of these numbers.
Sponges are unique in that they have no symmetry at all. Water passes through their bodies, which grow in whichever direction is convenient. All sponges (Phylum Porifera) live in water and the vast majority are to be found in the sea filtering plankton.
Humans often use the ‘skeleton’ of sponges when bathing, but it has been reported that some dolphins also use sponges. One population of bottlenose dolphins grabs a sponge and holds it over the nose when poking about in the sand, and this behaviour seems to be taught to young ones by their mothers.
Evolution of Symmetry
The earliest animals were probably bilateral, evolving a front end that moved through the water seeking food. Later (when there was plenty of plankton to be eaten) radial symmetry evolved in some groups as a good way of eating the plankton without having to use up energy moving about.
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