Roundworm progressive traits.

These are the first organisms to have a primary fluid-filled cavity that washes the internal organs. The liquid participates in the metabolism and creates pressure on the walls of the body, giving it shape. In the digestive system, such a feature is the division into sections and the appearance of the anus, which made it through. In the reproductive system, its simplification is progressive due to dioeciousness and the presence of sexual dimorphism. The function of the absent circulatory system was taken over by the fluid of the primary cavity.