This diverse phylum is made up of an estimated 1 million species of roundworms, many of which are parasitic. Nematodes have adapted to almost every possible habitat on earth, including marine and freshwater habitats, soils, tropical to desert to polar climates, and from very low to very high elevations. They are quite literally everywhere, and they outnumber us and all other animals in species count and actual numbers by a long shot! Species of these slender worms can vary greatly in size, from microscopic proportions to over a meter in length! Widely known roundworms include hookworms, pinworms, whipworms, and the species that causes trichinosis.