Secernentea are the main class of roundworms. Some Secernentea are parasitic, while others are not. They have excretory and digestive systems, but they do not have circulatory or respiratory systems. This class of nematodes are mostly found in soil, not in water. This class is also sometimes called Phasmidia because they have structures, known as phasmids, on their tail, making them sensitive to chemicals. The males usually have a single testicle. There are three subclasses of Secernetea: Rhabditia, Spiruria, and Diplogasteria (27).