Affinities of the Heterostraci (Agnatha)

From a study of the perforations in the heterostracan carapace and the impressions on the inner surface of the bony plates it is suggested that the Heterostraci in most features belonged to a structural stage from which it was possible to derive both the gnathostomes and the cyclostomes (including the cephalaspids and anaspids). However, the structure of the gills and the presence of a spiracle in the Heterostraci indicate that they were more closely related to the gnathostome stock.