Archiannelida

Archiannelida is a group, often classified as a class, of small, primitive annelid worms. They are characterized by a lack of external segmentation and resemble polychaete larvae. While once thought to be the earliest polychaete group, some members, like Polygordius, are now considered specialized due to their simplified features, such as the absence of setae and external segmentation. These worms typically have cilia for locomotion, respire through their skin, and exhibit internal fertilization. Polychaetes, to which Archiannelida are related, are marine worms known for their distinct body segmentation and parapodia, which are paired, lobelike outgrowths bearing bristles.1234