They look sort of like a sea anemone with longer waving arms. Mine are pink, but here's a picture of a green one or if that's too hard to see I've scanned a picture here and elsewhere in this article from A Golden Guide: Pond Life by Reid, library #574.92. I found out there that Hydras are tiny animals and really are related to sea anemones! They can actually be seen with the naked eye and some can get to be 1 inch long, although my biggest one is maybe 7 mm. They eat one-celled animals like protozoa and small crustaceans which they capture in their tentacles. They can even move about slowly on their `foot'. With only my magnifying glass I have been able to watch them reproduce by forming `buds' that looked like tiny hydras and eventually break off and become tiny hydras! I've never seen any in the pond but I know they are there eating from my `soup' and in turn being eaten by larger critters, like maybe dragonfly nymphs? I know that because my water in the polliwog tank came from my pond. Under the microscope they look really menacing, but they can't hurt us at all.