A phylum of microscopic (0.04–2.00 mm) pseudocoelomate aquatic animals belonging to the Lophotrochozoa and characterized by a crown of cilia at the head end. These are used in locomotion and in some species for feeding: the crown resembles a rotating wheel when the cilia are beating. Rotifers possess jaws and are covered with a layer of chitin (the lorica). There is no circulatory system and gas exchange occurs across the body surface. Some rotifers reproduce by parthenogenesis, existing only as females for many generations. Indeed, molecular analysis of asexual rotifers of the clade Bdelloidea indicates that they have maintained exclusively female lines for over 50 million years.

http://www.microscopy-uk.org.uk/mag/wimsmall/rotidr.html Basic descriptions and striking images of rotifers by Wim van Egmond