The Gastrotrichs.

Gastrotrichs are small (most commonly 0.1 - 0.5mm) transparent creatures with about the same order of bodily complexity as rotifers and tardigrades, but sufficiently different from them to warrant a phylum of their own. There are some 400 species inhabiting both salt and fresh water. The marine forms are found living between the sand grains of the intertidal zone, and the freshwater forms are found among the plants and algae of the pond's margins.

The name means "hairy stomach" and refers to the carpet of cilia which covers the creature's underside, enabling it to glide smoothly over plant and other surfaces. The gastrotrich's back can be covered with scales, or more commonly, an array of curved spines of varying length. Their bodies, whilst flexible, undergo little change in shape in the normal course of their activities.
Gastrotrichs are not quite as entertaining as rotifers and tardigrades, but almost.

Click for a diagram of a gastrotrich.