United States Fish and Wildlife Service list of threatened and endangered arthropods species may be treated as endangered or threatened if it resembles in appearance a species which has been listed and enforcement personnel would have difficulty distinguishing between the listed and the unlisted species; if the effect of this difficulty is an additional threat to the listed species; and if such treatment of the unlisted species would improve protection for the listed species. United States Fish and Wildlife Service list of threatened and endangered arthropods List of threatened and endangered arthropods, including insects, arachnids, and crustaceans under the United States Endangered Species Act as of October 2013. Key to Listing Status
Arthropod gills that were very different from those of arthropods. This reasoning implies that "Parapeytoia", which has legs and a backward-pointing mouth like that of the earliest arthropods, is a more credible closest relative of arthropods than is "Anomalocaris". In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids. In 2014, research indicated that tardigrades were more closely related to arthropods than velvet worms. Higher up the "family tree", the Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was