Nematoda
From Microcosm Aquarium Explorer
[edit] The Roundworms & Nematodes
This is one of the largest phyla of animals, found in freshwater, marine, brackishwater, and terrestrial environments. They are common parasites in domesticated animals, including tropical freshwater and marine fishes.
Crawling Caenorhabditis elegans hermaphrodite worm. Bob Goldstein/GNU
Known species: 80,000 Parasitic species: 15,000+
Image Credit: Wiki Commons/Goldstein lab
http://www.bio.unc.edu/faculty/goldstein/lab/movies.html