Lophotrochozoa

In order to catch more food, you develop a ring of tiny tentacles around your mouth which pull food in.  This is called a lophophore.

In a marine environment, it is very useful to be able to pull in food as it drifts past, or as you move through the water.  There is plenty of organic matter in the water, some of which is alive until you grab it.

Unfortunately, some of the things being grabbed, are your young.  D'oh!  Maybe it's time to toughen up again?

Grow a shell?
Stay shiny?

Platyzoa

You remain a small worm, with an incomplete gut, which makes a living in the safe crevices of the world.

Many of you become flattened in order to maximise the rate at which oxygen travels through your skin, as you have no complex internal organs to pump blood or exchange gases.  Some of you acquire a common name of 'tape worm' as a result of this shape.

Some of the crevices you hide in are inside other animals, and you become a parasite, causing disease and slow death, as well as damaging the brains and reproductive capabilities of your host.  But what do you care?  You've evolved to survive, and this tactic is working well for you.

Soft Protostomia

Your muscles develop, and you become a faster and faster moving little worm.  You remain small though, and are still ideal food.

You spread throughout the oceans, probably burrowing into the soft mud at the bottom and coming out to feed.

There are lots of other little worms around, with slightly different lineages since you were bilateria.  They will be competing with you, and you will need to out-compete them.  You can either get better at detecting and consuming food, or you can find a way to hang out where there is plenty of food and very little danger.

Bravely go out and find food.
Safely hide where the food is.

Enteropneusta

As you crawl along, you run the edge of your mouth along the bottom, taking in sand and dirt, and digesting anything useful which goes in with it.  Your long gut becomes longer and longer, to extract as much as possible from the detritus you find on the ocean floor.  Eventually, some of you become more than two metres long, all comprised of gut behind a muscular 'head' section.

For safety, you tend to burrow into the sea bottom, and then up again, making a U shape.  You stick your head out to feed, or stick your tail out to defecate.  When you live at the sea's edge, this makes makes characteristic 'casts' of sand at low tide.

The muscular head gives you a characteristic shape, which looks familiar and gives you your common name.  You are an acorn worm.

Pterobranchia

As you crawl along, you sometimes stop and rear your little wormy head up into the passing water.  A nice little plankton occasionally drifts into your mouth as a result.  A bigger mouth develops, which makes this more likely, and the edges of the mouth become frilly with cilia edgings, which wave in passing food.

You crawl less and less, and while you rear up your head, you dig your tail down to help you stay anchored.

When you create eggs, your larvae are free swimming until they find a good place to live, at which point they start to change shape, and excrete a glue which helps them stay still.  There you will stay for life, often in a small colony of similar, possibly even related, filter feeding worms called Pterobrachia.

Hemichordata

You develop a fold in your gut layer which differentiates to become a simple neural cord.  Now you can grow a longer body without one end losing touch with what is happening at the other.

You are an ocean living worm with a 'head' end which contains your organs, a mouth which sits behind that, and a 'tail' end which contains a long gut for extracting as much as possible from your food.  For food, you will eat anything you find dead on the floor as you crawl along, or anything living which drifts in as you swallow water.  Your nice long gut doesn't care.

Competition is tough, as always, and you need to collect more food to win.  You can specialise in crawling, finding dead stuff on the floor, or stick your bum end in a hole and stick your head up to catch whatever is swimming past.

Head up, bum down.
Crawling along.