Basket Stars

Each of your five arms develops arms of its own.  Those arms develop even smaller arms.  This repeats until your arms fill the gaps between them like a filigree. When you hold them out, they look like a fine mesh basket, which gives you your name: a Basket Star.

Beautiful, but mostly unseen, living in the deep cold waters of the northern hemisphere of the planet,  you crawl across the ocean depths, hiding during the day, and feeding during the night.  You are a pitiless hunter, grabbing krill - small swimming creatures -  in your basket and eating them.

Holothuroidea

Getting bigger, longer, musclier, and thicker, you have a long, efficient gut, and can get all the nutrients out of your food.  You live through several stages in your life now: tiny worm, little star, bigger star, until you become so thick and muscled that you tilt back and begin to look like a worm again, but a huge, thick worm, the size and shape of a common vegetable: you are a sea cucumber.

You still have radial symmetry, but unlike your sister species, your axis reverts back to horizontal, and you crawl or float or swim along with two 'legs' down, two to the sides. and one on top.  You secret a poison called holothurin which keeps you safe from many predators.  You mostly eat whatever detritus you find on the bottom, or whatever floats past you in the current.  Different species range from a few millimetres long to a whole metre long, and you live throughout the seas, although you are at your most dominant in the deep sea.

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?

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.

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.

Chordate animals

Now that you've developed into a tube, some parts of your body are a long way from some other parts.  You already have nerves and sensory apparatus, but it would be useful to have some decent plumbing to get signals from one end to another.

As you develop, you are comprised of three layers of cells - the endoderm, mesoderm and ectoderm, which become, roughly, skin, muscle, and gut respectively.  You need to make a new kind of tissue to become a communications hub.  It can develop from either the mesoderm (muscle layer) or the endoderm (gut layer).

Muscle.
Gut.