OpisthobranchiaOpisthobranchia

This is a shelf, not a branch of the family tree. Opisthobranchia was the old name for the sea slugs, everything that looked like a snail with the shell gone or going, and the name says where the gills sit, behind the heart rather than in front of it. Then DNA showed that these animals do not all descend from one ancestor that gave up its shell, and taxonomists retired the group. It survives as a heading because what it collects is something you can see for yourself: a shell caught in the middle of vanishing.

Run through the group and the stages are all there. A bubble shell still has one, but thin and too small to pull into. A headshield slug carries the shell buried in its own flesh and uses a hardened wedge on its head to plow under sand without packing its gill chamber with grit. A sea hare keeps a flat plate of shell under the skin of its back, which you can feel through the tissue like a fingernail. The nudibranchs, on their own page here, finished the job.

Sea hares gave science something else. Their nerve cells are the largest known in any animal, big enough to see without a microscope and to record from one at a time, and it was on a sea hare that researchers first followed what physically changes inside a nerve cell when an animal learns something. That work took a Nobel Prize in 2000. A soft brown slug in the seagrass is one of the best understood nervous systems in biology.

Handle one roughly and it answers with ink. The purple comes from the red algae it has been eating, and it is fired together with a second, sticky secretion that clogs the taste and smell receptors of whatever is attacking, so the slug is not so much hidden as temporarily unfindable. Fifteen species are listed here: sea hares in the seagrass, headshield slugs cutting trails across sand at dusk, side-gilled slugs under rubble in the daylight.