Strange snail-like creature

I found this picture of a strange creature somewhere, but I don't know what it is? It looks kind of like a snail or a slug, but it lives underwater. Here's a picture:

It is a Sea Slug.

Search on google for Atsushi Ono and you will find that he appears to be somewhat of a sea-slug fan.....




Someone's got to be, I guess....

looks kind of like somethings larvea, like an insect or something

Ooh. I see. Thank you. I just looked up the website and there are a lot of stranger sea slugs. O_O;

Haha, show'd the picture to my mom and got this response: "Well, thats a snail thats a crab in the middle!"

Nice little fellow though - Looks more like a tree-equator-living insect than something beneath the watersurface.

I've watched alot of discovery channel, but I've never seen a sea slug like that before. That bulge in the center is very, very bizaare and not like anything I've seen before.

Tell you the truth I think that this is a sea slug. I've done a project about these things before and this looks similiar to some of the ones I've seen. They are really cool and odd creatures really. Actually when I do look at the middle i can see two eyes and what seem to be crab claws. I'm not saying that it is a crab in the middle but I could see why someone may mistake it to look like a crab, maybe a camouflauge/defence mechanism?

Ok guys, this is what I have found after about a full night of searching.
If you right click on the picture and check the filename it says Gnathia. That's where I started. I went to Google image search and typed in "Gnathia" and I came up with this page. It's all in Japanese so I went back to goole and used their auto-translate feature and this is the translation I got...

1 kind
of グナチア Gnathia sp.

The arthropoda which very send strange life history. At the time of the larva it is the vampire which lives upon to the bottom-dwelling fish, but when it becomes the adult, the mouth-parts degenerating, the 摂 feed stops doing. So, as for this larva. The crab we would like to see it is visible, but whether or not it is mimesis, it is unclear. This companion is known about 70 kinds, it seems, but the own country product only is verified 1 kind. With, it was written on the old data.

So, back to Google I went. This time I typed in Gnatia Arthropoda with "parasite" in quotation marks. After sifting through a bunch of junk that I could not read I came to this site.

So, there is a reason it looks kinda like a crab... It is a crab larvae that attaches itself to fish like a leech(which would explain the "bulge in the middle").