Historically, a close relationship between eurypterids and xiphosurans (such as the modern Atlantic horseshoe crab) has been assumed by most researchers. Several homologies encourage this view, such as correlating segments of the appendages and the prosoma. Additionally, the presence of plate-like appendages bearing the "gill tracts" on appendages of the opisthosoma (the "blatfüssen") was cited early as an important homology. In the last few decades of the nineteenth century, further homologies were established, such as the similar structures of the compound eyes of "Pterygotus" and horseshoe crabs (seen as especially decisive as the eye of the horseshoe crab was seen as possessing an almost unique structure) and similarities in the ontogeny within both groups. These ontogenetical similarities were seen as most apparent when studying the nepionic stages (the developmental stage immediately following the embryonic stage) in both groups, during which both xiphosurans and eurypterids have a proportionally larger carapace than adults, are generally broader, possess a distinct ridge down the middle, have a lesser number of segments which lack differentiation and have an underdeveloped telson.