As a larval thecostraca, you swim in the currents of the sea, as zooplankton, picking up microscopic fragments of edible matter that passes you, one of the smallest filter feeders there are. Millions - no, billions - of you are eaten by animals such as Baleen whales, the biggest filter feeders of them all. Those of you that are lucky grow bigger, and moult your hard shell for a new, bigger one, and repeat.

Once you’re big enough to be eaten one at at time, you have a problem. Luckily, you do already have the solution: that hard carapace.

All you need to do is make it even bigger and harder. Not a problem. The downside is that this makes it impossible to walk, with the result that adult thecostraca move less and less - initially you hold onto a rocky object with your frontmost legs and wave your back legs in the water to collect food, but over time you develop a proteinous glue to help your grip.

Eventually that glue becomes so strong that you can glue yourself down permanently[1] and collect food that passes by in the water currents by flexing open the top section of your carapace, which now resembles a shell.

You do still have predators, many of them, but by reproducing frequently, living inside a thick shell, and growing together in huge groups that make it hard to get at any individual, you survive in vast numbers into the present day.

You are a barnacle.

[1] Of course, this also leads directly to another problem… reproduction. As thecostraca become increasingly immobile as evolution takes its course, the, err, “meeting” of a mummy thecostraca and a daddy thecostraca becomes a problem.

As always, evolution has provided: male thecostraca have the biggest penis to body ratio of any animal: up to eight times the length of their main body. You can imagine the rest.