You settle on five pairs of legs, enough to make you pretty fast, and pretty good at swimming too. When your larvae are small, they float in the water as zooplankton eating microscopic bits that come near, then at a certain size you swim to find your food, then if you are lucky enough to get really big, you sink to the bottom under the weight of your plates and run around.

There’s a lot of good food just floating around though. Might be best if you never grow that big, and just stay at the swimming stage?