Echinodermata

Once your larval form finds itself in a good location, it decides to stick around.  Literally.  It holds on to the bedrock, in the deep ocean, and waits for food to pass by.

Many species behave like this, but they all started off with a body shape which was suitable to filter feeding.  The most efficient shape to filter food from seawater, is a fan shape.  You are a tubular little worm, and it doesn't really work, although if you curve your body and look at it from one side, it'll do.  But this is evolution: you can't go backward, you have to do the best with what you have.  And when you do your best, something amazing can happen.  In this case, something really amazing.  So amazing that the great Charles Darwin described this as "the great mystery", although, like everything in evolution, it's simple when you know the answer.

One side is preferentially used to hold on to the rock.  The other reaches up to search for food.  Once you are in position, or 'sessile', therefore, there is an advantage in one side growing bigger and bigger, but the other can stay pretty much the same size.

There is now an evolutionary advantage in genes which make one side grow bigger than the other.  It happens to be the left side which holds on, and the right side which undergoes metamorphosis.

The right side grows, and as it grows, it adapts into five large sections, each of which grows outward to reach food, until it becomes something that looks like an arm.  Your arms are covered with tiny 'tube feet' which grip food by suction, and pass it to your mouth, now located at the centre.

Your left side is almost entirely re-absorbed during this process.  The result is an animal which no longer has bilateral symmetry, but radial.  You are a flower shaped eating machine, on top of a small stem.

There's a plumbing problem here though: your expanding right side curls right around your mouth, and the little tube feet on the upper surface pass the food to it, but where is your bottom going to go?!

If your right side grows around below your anus, then you'll end up with a U shaped gut, and mouth and anus on the same side of the body. If it grows around above your anus, you'll get an I shaped gut, and mouth and anus on opposite sides.  What's it to be?

U?
I?