Help identifying what this is in Crayfish gills?
My best guess would be Trematoda which is a fluke worm that infects mollusk. I would think this is the larval stage of cercaria which is free swimming and develops in the mollusk and then moves on to infect others.

Here are some different images of a cercaria:

 



In this YouTube video you will see what is known as a "Rat King" which is just a ball essential of Trematoda. Here is a screenshot