Fertilization is internal. After fertilization, the life cycle may follow one of two paths, either direct or indirect. Instead the larvae develop inside the egg so that the infective stage consists of an egg with a larva inside. In the indirect life cycle, the larvae develop into an infectious stage in an appropriate intermediate host. Roundworms are cylindrical, tapering animals with simple bodies consisting of an interior gut and a muscular outer wall, separated by a fluid-filled cavity called a pseudocoel, a type of coelom.
Roundworms show anterior cephalization, a major evolutionary advance.
This is described on the flatworm page.