A flower shape, which collects passing food, with a stem, sometimes short, sometimes long, and a gut that goes right through you, is a pretty good shape.
Your 'stem' grows longer and longer, so you can reach more food, and eventually you flip right over like a wilting flower, and end up 'face down' on the sea bed. You can use your tube feet to move around now, as well as eat. You already have hard scales on the 'back' of the flower, so you are reasonably well protected, and you grow bigger, and bigger. Except during your larval phase, you no longer look even remotely like the worm you were, just a few choices ago, and you don't even need the stem any more.
In evolution, anything which is not needed is just extra cost, and so the genes which create the stem are selected against, and you become a free-roaming little star-shaped animal, with an armoured back, eating whatever you walk over, and excreting the waste through an anus on top of you.
Now that your mouth is at the bottom, and your bum is at the top, it might even be more efficient to just dispense with that too, and let any waste fall back out of your mouth. Sounds awful, but efficiency is often king in evolution.
Leave my bum alone!
Don't need a bum.
Evolution starts with the Last Universal Ancestor.
You can find out what happened before evolution here.
You can also short-cut to Land plants, or Animals
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