Interesting critter. Are you sure it is an aquatic organism and not an unfortunate terrestrial? The reason I ask is the prominant, thick, antennae on the specimen. These disqualify Chadk's photo suggestions since they have prominant mandibles (pincers) on their heads, not antennae. Could you tell if the appendages on each of the segments were legs or gills? You can also note on Chadk's photos that each segment has gills and only one on each side of each segment. They are "insects" and therefore only have six pairs of leggs. Hard for me (a retired biologist) to get any more specific on the basis of the photos and nt having a bug book with me.