Ascaris
Ascaris species are very large (adult females: 20 to 35 cm; adult males: 15 to 30 cm) nematodes (roundworms) that parasitize the human intestine. A. lumbricoides is the main species involved in human infections worldwide, but pig-derived Ascaris (often referred to as A. suum ) can also infect humans. These two parasites are closely related and hybrids have been identified; thus, their status as distinct and reproductively isolated species is a controversial topic.