Quote:
Originally Posted by whswhs
It's actually a logical design. If you eat other living things there's a fair chance you want to move around looking for things to eat. If you do that, it's efficient to have your distance sensors clumped near the front, and to have big nerve ganglia there. Nonbilateral symmetry seems to go with having a pretty minimal nervous system, and then I don't think you get sapience. Nor rapid movement, though lots of animal phyla are slow-moving.
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Front-back orientation and degrees of symmetry are very different questions. There's nothing stopping you from having higher-order symmetry and being elongated along the axis of symmetry with front-clustered sensory and processing organs.
Bilateral symmetry
does have a distinctive benefit in supporting top vs. bottom differentiation (front vs. back for us weird uprights, but that's much later) and perhaps providing simplification for appendages that cooperate between symmetry units. The former, at least, seems like something rather useful for terrestrial life - but the bilateral clades were bilateral long before being terrestrial, so that's not
why they did it.
EDIT: While molluscs are basically bilateral, I think, it's easy to imagine something on the squid or octopus sort of body plan that actually has 4 or 8 fold underlying symmetry. That's not so helpful of course when we specify land-dwelling.