I've been struggling to capture a decent image of a gastrotrich for quite a while, but this afternoon this obliging example (Chaetonotus sp., I think) paused just long enough in my field of view for me to record it. Gastrotrichs are miniature aquatic equivalents of the roadrunner, always on the move - although in this case propelled by a layer of beating cilia on their underside (gastrotrich means 'hairy stomach') rather than legs. This one seemed to be feeding. That circular mouth, surrounded by a ring of tiny teeth, and the muscular pharynx that it leads to ingest just about anything that the animal collides with and that's small enough to enter the gap. The outer covering of cuticular spines give this gastrotrich a fearsome appearence and if you are smaller that it is (i.e. less than about a quarter of a millimetre) the sight of one of these high-speed hunters hurtling towards you must be a nightmare - not that pond life suffers from nightmares; sorry, lapsed into anthropomorphism there. Been watching too many horror movies.
This specimen came from the edge of a Phragmites reed swarm, amongst the rotting debris in shallow water - a favourite habitat for gastrotrichs. You can see here how agile they are, capable of turning in their own length through 180 degrees. Those two appendages on the tail are adhesive organs. Gastrotrichs do sometimes rest and when they do they glue themselves to something convenient with adhesive secreted from the tips of those appendages. When they are ready to go again they secrete a releasing agent - that, and the glue, which both work under water, must be very interesting substances.