Brachiopods

Growing a pedicle allows you to attach yourself to a convenient location when you find yourself in one, and become 'sessile' or permanently stuck to one place.  

You can stay there, opening your shell and filter feeding when it's convenient, and closing it again to hide inside when there is a predator around.  You release many, many tiny copies of yourself, which float around with the currents until, in their turn, they find a good place and stick themselves there, where they grow thicker and thicker shells and make more copies.  

Interestingly, since the Permian-Triassic extinction event, which didn't quite wipe you out, you haven't grown back as rapidly as a closely related species, the bivalve molluscs.  As a result, they appear to have dominated the upper, richer, waters, and you are mostly found at deeper locations.

It's a simple, relatively low cost way to make efficient use of free resources, and you are found in deep seas all over the world.

Hyoliths

You have become a Hyolith.  There were many species of hyoliths, but all took the form of a long conical or triangular shell, with a shell cap, and two helens which enabled them to stand up to feed with a crown shaped filter mouth.

You are first found in the early Cambrian period, over 500 million years ago.  However, your numbers appear to dwindle during the Permian period, up to 250 million years ago, possibly due to increasing acidity of the oceans making it hard to make enough shell material for your lifestyle.

At the end of the Permian period is the Permian-Triassic extinction event - the biggest extinction of species in the history of the Earth.  The cause of the P-T extinction is still debated, but we know that 90% of species didn't survive it.

You, already declining, were one of them.

Dorsally shelled lophotrocozoa

With a shell above, and a shell below, you're safely protected, and can open and close to permit feeding, and to hide.  When the shell is open, you filter feed happily. 

As 'babies' dorsally shelled lophotrocozoa float around until they have accreted enough material in their shells to sink to the bottom.  At the floating stage they are called plankton.  Ideally, they eventually end up in places where there is a slight current to carry food to them.

If you land on a nice rocky shelf, you can build nice big shells and reproduce happily and safely for a long time.  You don't want to get swept away though.

If you land on soft mud, this isn't going to work so well.  You may get covered with silt, over time, which makes filter feeding difficult. 

On rock, you can ensure you don't move by poking a sticky bit of your body - a 'pedicle' - out through your hinge and gluing yourself down.  On mud, you need stilts! These take the form of extending your bottom shell into a long cone, and growing two long 'legs' near the front called 'helens'. Together these form a triangular prop, enabling you to lift your mouth up out of the mud, and continue to feed. 

Obviously stilts on rocks are no good, and sticky pedicles on mud don't work either. 

Helens or Pedicles?

Shelled Lophotrochozoa

A shell is a useful addition.  There's far too many things out there bigger than you willing to gobble you up.  The way you make yours is fascinating.... You develop a special set of cells on your outside called the mantle.  These contain proteins which extract calcium carbonate from sea water, then express it on their outer surfaces (with a little bit of protein to provide a scaffold).  This makes a hard surface just outside your body, which makes you a little less attractive as food.

As you grow older, this layer grows thicker and thicker.  However, it's a bit of a restriction, as you can't actually flex this stuff - it's just too hard.  Over many generations, a pattern emerges which gives you the best of both worlds - protection and movement.  It is a simple hinge, which sits between two separate shell structures.

The hinge allows you to open the two parts of your shell to feed, and close it when there is danger.  Perfect!

There are two effective ways to make the hinge.  Your two shell cases can be:

•  on your left and right sides
•  on your top and bottom sides