Roundworms are in the phylum Nematoda. They are the most common worms in the sea.
The Phylum Nematoda consists of the species commonly known as roundworms.
The sexes are usually seperate but there can be a hermaphrodite roundworms.
Nematodes- (Phylum Nematoda "thread species") Also known as round worms, there are between 10k and 25k species. They live in sediment or the tissues of other organisms.
Phylum Nematoda on Pinterest | Anatomy, Fresh Water and Body Shapes