I discovered this beastie in a worm tunnel in my aquarium a week ago. It sort of looks like an amphipod with eggs, but stays in the tunnel like a worm, only sliding up to the surface of the sand, then down again every so often.The legs at the top look like they belong to a spider. At the bottom end, almost transparent appendages flutter constantly, again like an amphipod or the shrimps. I think there may be more than one in the tunnel. They, or it, don't match any of the green amphipods in the tank; it's longer and thinner, and leggier.
I've been spending a few minutes every day since, trying to get a better photo, with no luck. I'll keep trying.