Digestion and Evolution
Imagine an assembly line for assembling things instead of
putting them together. Compare the earthworm's tubelike digestive system
to such a "disassembly line." What is the advantage of a tubelike
digestive system over a saclike digestive system?
There are several. One is that you will not mix your waste with your
undigested food. Second, you can concentrate on specific digestion
without interfering with other digestive processes. These two reasons
alone are very significant justification for the one way tube.
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Update: June 2012