They have a total of seven eyes, including two lateral compound eyes, a feature found in no other living chelicerates.  
The family of this organism contains four extant species, three native to Asia and one native to North America, which are also the only extant species in its order, Xiphosura.  Its distinctive caudal spine is used for steering and uprighting itself, and they possess five pairs of (*) book gills.  They have a total of seven eyes, including two lateral compound eyes, a feature found in no other living chelicerates.  Their blood uses hemocyanin as its oxygen carrier, and an extract of its blood amebocytes is used in the gel clot procedure for bacterial endotoxin testing.  More closely related to the arachnids than to decapod crustaceans, for 10 points, name this arthropod which has existed essentially unchanged since the Paleozoic era.
The family of this organism contains four extant species, three native to Asia and one native to North America, which are also the only extant species in its order, Xiphosura.  
{horseshoe crab} or family {Limulidae} (2) [AU]