Selection pressure makes you get bigger and bigger, because being big makes you less likely to be eaten. You develop another kind of cell which is neither a specialised feeding cell for the inside surface or a specialised outside cell for the outside surface, but a specialised inside cell to fill you up.
Do you remember how we described the big ball of cells as a dented ball? Despite the new kind of cell, the dents just get deeper and deeper. Eventually, they get so deep that the dent becomes more than a mouth, it actually punctures through the back of the animal, and the animal becomes a tube.
This tube shape will be very efficient, as you can keep pushing food in at one end and let what you can’t use fall out the back. Obviously, you only want to expend the energy to develop sensory, food-pulling cells at one end, as it would be inefficient to have them at both, so the next choice is simple: pick a direction. Is the first dent going to be the mouth or the anus?
This tube shape will be very efficient, as you can keep pushing food in at one end and let what you can’t use fall out the back. Obviously, you only want to expend the energy to develop sensory, food-pulling cells at one end, as it would be inefficient to have them at both, so the next choice is simple: pick a direction. Is the first dent going to be the mouth or the anus?
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