On land, snails and slugs -- the Gastropods -- are confined to terrestrial prison, but in the ocean, they are free to shed their shells and fly. These are the sea angels, the sea butterflies, and the sea elephants -- and probably quite a few more I'm not aware of.
For instance, this slinky and mysterious creature is a heteropod ("different foot"), or sea elephant:
[Heteropods] are phenomenally cool mollusks that have forsaken their snail shells to swim naked and free in the ocean like vicious little hippies. They look for the other pelagic creatures from which to take bites using their saw-like radulas at the tip of a Futurama-esque eye-stalk (but it's not — the eyes are at its base). The larval forms still possess coiled mollusk shells, but they lose them when they become adults. They also possess a single “dorsal fin” — which is actually totally inaccurate because it is really ventral (stomach side — they swim “upside-down”) and was originally the mollusc’s foot – which they undulate and paddle about with. For some reason, when moving, they remind me of Sir Hiss tooling about in that ridiculous balloon at the tournament in the 1973 Disney “Robin Hood” (see 2:15 here, and :20 here for the Sir Hiss Version). Some species possess a sucker on their “fin” which the heteropod no doubt uses to hold its prey still while it savages it alive.
Then there is the sea butterfly.
Sea butterflies, the Thecosomata ("encased bodies" also sometimes called "pteropods"), possess fluttery, flappy wings ("parapodia") that are actually yet another modified gastropod foot -- the slimy, crawly part of a snail. They use these wings to haul themselves and their shells -- when they have them -- through the water column. They seem to be omnivorous, but spend at least part of their time making and dragging a mucus net through the water with which to snag food. They don't seem to be sentimental about their gooey blankets, though. If disturbed, they'll drop them and flap away.
Sea butterflies are also, strangely enough, one of the creatures scientists are most concerned about as the oceans acidify due to rising carbon dioxide concentrations. Here's a clip from a BBC documentary (the new climate change one?) that shows both sea butterflies' flappy wonderfulness and explains their sad expected fate.
Strangely enough, sea butterflies are the sole menu item for some species of sea angels (the Gymnosomata, or "naked bodies"), their possible close relatives in the informal gastropod taxon called "pteropods". These would be the flying slugs. And I imagine if I were a slug, this is what I would dream of evolving into one day (torrid slug love scene in "Life in the Undergrowth" notwithstanding). They actually do hatch with tiny shell on their backs, like many heteropods, and then sever the connection to it and cast it aside not long into their development. In this way, you might say they have a vestigial shell that reveals their gastropod ancestry just as whales have vestigial leg bones that reveal their terrestrial ancestry.
Here's an excerpt from a National Geographic documentary (perhaps the recent one on climate change?) that shows sea angels snacking on sea butterflies nicely.
I bring all this up because two of these three strange gelatinous mollusks (a sea elephant, or heteropod; and sea butterfly, or Thecosomatan, slides 1 and 4 respectively) are highlighted in a beautiful slideshow of deep sea oddities taken by Duke University optical biologist Sonke Johnson over at Duke Today. In the slide show are other delights as well, such as what a jellyfish parasitized by amphipods looks like (I've never seen that before), a beautiful plant-animal living fossil called a stalked crinoid, a gawky mantis shrimp larva which takes the concept of eye stalks to a whole new level (is it possible to overdo it, really?), and finally, what a baby flounder looks like before its weirdo eye migrates to the other side of its head (its pretty!).
Hit the button in the bottom right corner of the slide show menu to make the art as big as possible, and make sure to take time to study the photographs for a good long while both before and after reading the text. I'm often guilty of reading the captions and barely glancing at photos in slide shows. Don't make that mistake. Savor.