Roundworms are in the Nematoda phylum. There are over 25,000 species of discovered roundworms. They often latch onto the skin of dogs and other household pets. They can also infect humans by invading their small intestine. Trichinosis roundworms (Ascaris lumbricoides) have a very small, primitive muscular system. The few muscles that they have allow them to move from side-to-side and lift them off the ground, much like the Oriental lung fluke. They also have a digestive system that serves as a mouth and an anus. All of their muscles are bilateral, except for the muscles in their heads which have radical symmetry, and form underneath the hard epidermis. The muscles are perfectly aligned down the length of the body, so that the roundworm can move quicker and to the side, allowing it to obtain food, find a mate, and escape predators.