"This mesmerizing, wiggling worm is the back end of a live tapeworm hanging out of the intestine of a bird. Tapeworms don’t eat the food in the intestine. Adult tapeworms do not have mouths and absorb nutrients through their body surface as it passes by in the intestine." - Anna Phillips, Curator of the U.S. National Parasite Collection Smithsonian's National Museum of Natural History #ParasiteWeek2021 | Smithsonian National Museum of Natural History

This is a marine worm, Dinophilus, and it’s parasitized by unicellular organisms called Grellia. You can see the parasites as blue, banana-shaped structures in this polarized view. . Dinophilus are a few millimeters in length and inhabit all marine environments. They eat algae and mind their own business without bothering anyone. However, the large population I had in one of my fishless tanks were all infected by a species of unicellular organism. . When I checked an odd-looking Dinophilus under