Asymmetrical animals
Virtually all animals on earth ranging from insects through to swimmers, fliers, mammals and reptiles, have a symmetrical body plan. In other words legs and wings come in pairs, as do most sensory organs. The mouth is centrally placed, etc. http://en.wikipedia.org/wiki/Symmetry_in_biology#Bilateral_symmetry
Would there be any reason why a creature might evolve a non-symmetrical body plan?
Really I'd be interested in a world where the dominant and/or most common body plan was asymmetrical in some way, and a way in which that could arise.
To help inform the debate there is a list of example animals on earth which are asymmetrical here: http://en.wikipedia.org/wiki/List_of_animals_featuring_external_asymmetry (Thanks to Pavel Janicek for the link).
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1 answer
You need to figure out what environmental reason there would be to not be symmetrical.
The asymmetric earth creatures I can think of are crabs and lobsters, where one claw is for crushing and the other for cutting. Lobsters for instance don't have much of a mouth, and no teeth, so they use their crushing claw to break open an oyster, and their cutting claw to snip it into tiny pieces that they can throw at their mouth.
A fictional example of asymmetry are the Moties in "The Mote in Gods Eye" by Larry Niven and Jerry Pournelle.
These creatures had a caste system where their body plan would give them an advantage for their jobs. Workers had two arms on one side with delicate manipulating hands and a massive gripping hand on the other side.
On a side note, this led to asymmetric points of view "On one hand you have A, on the other hand you have B, on the gripping hand you have C".
All you need for a world where symmetry isn't the norm is a reason why this would give an advantage at an early stage of development, since any further adaptations would follow the same pattern.
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