If you walked down our local canal, you would see strange growths on some of the sticks underwater. Before you hit the panic button, you may want to know out that these are non-alien animals called Bryozoa.
Bryozoa are small coral-like animals that build the strange pods in order to filter out the food they need in the water. The majority of bryozoans are marine, consisting of several thousand species. But one class, the Phylactolaemata, is found exclusively in fresh water. (These are the strange pods that you would see in your streams.) Bryozoa have also been seen in fossils dating back to 354 million years ago.
Recently we got one from our local canal to study, though we had some trouble getting it out. We had to find one that was small enough to fit in our jar, and close enough to shore to reach. (We didn't know how deep the canal was!) The colony we picked was one of two attached to a loose branch that we were able to pull on shore. Then we had to break it off to fit it in the jar. (Mom forgot her Swiss Army knife.) We made sure to fill up the jar with the water it came out of -- plus an extra container -- and carried it home carefully.
Up close, we could see that the pod was made up of clear jelly-like substance with spots all over the surface. The next day we took samples from our jar and looked at them under the microscope. First we looked at some jelly that was stuck to the stick, where the larger colony broke off. It was yellow and cloudy, but we couldn't see anything moving or otherwise interesting.
Then we used a pipette to suck up one of the small, dark green dots around the edge of the jar. (See slide on left.) Under the microscope we could see a dark center, a lighter ring, and little hooked feet sticking out all around. (Closeup on right.) This is a statoblast, which is used by the Bryozoa for reproduction. The statoblast can survive the winter and grow a new colony in the spring. According to this site, the jelly and everyone attached dissolves in cold water.
We're wondering if the yellow things in the middle are the actual polyps. Here is a website with great microscopic photos and videos of Pectinatella magnifica. What do you think?