Relations to Other Phyla

Annelids have several relations to other phyla, such as the Arthropods and the Molluscs.

Arthropods are considered the closest relatives of the Annelids, due to their similar/common segmented body structures, but seem quite different otherwise. It is assumed that they undrwent convergent evolution to create the grouping of Articula. The only differences between them are that Arthropods have an open circulatory system, a respiratory system (with a trachea, gills or book lung), excretory glands, normally sexual activity, and an exoskeleton.

Molluscs share with the Annelids the trocophore larvae stage, as it is present in both groups. It's due to this that the Annelids and the Molluscs are grouped together as the Trochozoa. The Molluscs and the Annelids are different only in the fact that the Molluscs have an open circulatory system, a respiratory system, have a well developed brain, and have a nerve network in their body walls.