What organ systems first appear in annelids?

For the first time, in terms of complicating the organization, a closed-type circulatory system appears in annelids. There is no heart in it yet, and several thickened vessels perform its function. A special fluid flows through the vessels – hemolymph. The simplest version lacks respiratory pigments, although some species already have them. There are no special cells in the circulatory system of annular worms.