Editing Chelicerata (section)

===Respiratory systems===
These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both [[book lung]]s, which deliver oxygen and remove waste gases via the blood, and [[Invertebrate trachea|tracheae]], which do the same without using the blood as a transport system.<ref name="RuppertFoxBarnes2004ArachnidaGen">{{harvnb|Ruppert|Fox|Barnes|2004|pp=559–564}}</ref> The living [[horseshoe crab]]s are aquatic and have [[book gill]]s that lie in a horizontal plane. For a long time it was assumed that the extinct [[eurypterid]]s had gills, but the fossil evidence was ambiguous. However, a fossil of the {{convert|45|mm|in}} long eurypterid ''[[Onychopterella]]'', from the Late [[Ordovician]] period, has what appear to be four pairs of vertically oriented book gills whose internal structure is very similar to that of scorpions' book lungs.<ref name="BraddyAldridgeEtAl1999LamellateBookGills">{{citation |author1=Braddy, S.J. |author2=Aldridge, R.J. |author3=Gabbott, S.E. |author4=Theron, J.N. |name-list-style=amp |year=1999 |title=Lamellate book-gills in a late Ordovician eurypterid from the Soom Shale, South Africa: Support for a eurypterid-scorpion clade |journal=[[Lethaia]] |volume=32 |issue=1 |pages=72–74 |doi=10.1111/j.1502-3931.1999.tb00582.x|bibcode=1999Letha..32...72B }}</ref>