Myxobolus cerebralis
The kind of M. cerebralis that can attack fish is called a "Triactinomyxon", and there is a picture of it on the right.  There are three "tails" that are each about 200 micrometers long, and another piece that is about 150 micrometers (μm) long.  That piece has 64 cells in it. There are also three harpoon-like weapons that can attack the fish. They shoot into the fish, making a hole that the 64 cells can go into.  Those cells make more cells that go to the cartilage near the fish's brain.