Annelids

Taxonomic affinity seeAlso The group had been variously assigned to the echinoderms, barnacles, annelids and mollusks, before the discovery of a fossil preserving soft tissue allowed a firm classification to the annelids, in 2008.

The exact relationship to crown group annelids are still unresolved, but some characters indicate a relationship to Aphroditacean annelids (Vinther et al. 2008).

Although some species lack a blood vascular system, where it is present, it resembles that of other annelids.

Annelids such as Serpulites are common fossils in some horizons.

The brain is relatively large, compared with that of other annelids, and lies in the upper part of the head.

The frontal glands or organs of flatworms vary a lot in structure, and similar structures appear in small marine annelids and entoproct larvae.

The Journal of Neuroscience, 17: 459-469 Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods.

The parapodia appendages found in annelids therefore correspond to the stump feet of the velvet worms.

This perspective was expressed paradigmatically in the statement by the French zoologist A. Vandel: :Onychophorans can be considered highly evolved annelids, adapted to terrestrial life, which announced prophetically the Arthropoda.

This places the annelids and Panarthropoda in two very different groups: the former in the Lophotrochozoa and the latter in the Ecdysozoa.

Unlike other annelids, adult echiurans have no trace of segmentation.