I assume you are referring to an Aquatic Sowbug. However, I believe it to be a Water Penny Beetle larva, probably of family Psephenidae, subfamily Ebriinae.Freshwater isopod?
I would have said rolly polly (pillbug).
Great job, Mark.I'll still stick by my original ID of freshwater isopod or aquatic sowbug. If you look at the head it appears to have an antennae coming out, also the head is showing which it wouldn't be with a water penny and at the extreme rear end of the abdomen it appears to have a cerci coming out. Also down at the 8th and 9th abdominal segment it tapers in. All these are indicative of a freshwater isopod.