How can you not think of jellies, when given the word glassy? This a crystal jelly (Aequorea sp.), one of the clearest jellies I can think of. I think this jelly is pretty neat because it is the jelly that GFP (green fluorescent protein) was isolated from.
I also really dig the way these guys eat. The batch of tentacles in the center surrounds the mouth. Unlike other jellies, the mouth can open really, really wide so that it can swallow and eat other jellies that are up to half its size. I've never seen another jelly that could open its mouth this wide, or even open its mouth at all. And they look pretty neat all full of food, too.