parasite

The tail end, I think.

This is the worm, actual size, on a microscope slide. the biggest one that I saw was about 1 cm long. this one's about 6 mm long

The front end of it. It has a straw-like feeding appendage, and it was actively poking around (looking for food?) with that end.

A worm that I found in my aquarium a while ago. I'm not sure what it is, but it might or might not be a parasite.

Another view of the head.