This link leads me to believe it is a Cupelopagis vorax, but I've seen other pictures that look quite different so I don't know if I've got the ID correct.
It is certainly interesting to watch. It holds a balloon-like membrane open until some small creature enters, then it pulls it closed like a purse seine fishermans net. In the upper picture you can see this occuring around some unfortunate small creature.
In the lower picture I have greatly magnified the small section shown in red on the inset. (In the small inset photo the membrane "trap" is fully opened toward the lower right. It's like a big scoop, with the lower edge protruding much farther than the narrow upper edge). In the large second picture you can see some interesting structures. The sensory structure on the left of this picture is outside of the "ballooned" membrane, and is very similar in appearance to structures I've seen on many other rotifers. On the right side you can see a "spray" of fine structures. These are actually located within the opened "trap" (the small DOF and semi-transparent nature of the membrane allows you to see through it). My guess (purely a guess!) is that they enable the rotifer to sense vibrations in the water caused by a creature that has entered the trap... it certainly knows when something has entered, even when the victim has not come into contact with the membrane.
The rotifer appears to be very sensitive to "pressure" vibrations caused by other creatures. It will sit completely motionless, but if something approaches it will slowly turn the opened trap up to about 15-20 degrees toward the potential meal.
Upper image 20X objective, lower image 100X objective. 2.5X photo-eyepiece. Canon Rebel 350 XT. Brightfield with electronic flash.
