With regard to the =affinities= of the group it is now almost universally accepted that they are Arachnids. The chief features in which they differ from other Arachnids are the presence of gills and the absence of Malpighian tubules, both being features associated with aquatic life. As long ago as 1829 Straus-Dürckheim emphasised the points of resemblance between the two groups, and although the view was during the middle of the last century by no means universally accepted, towards the end of that epoch the painstaking researches of Lankester and his pupils, who compared the King-crab and the Scorpion, segment with segment, organ with organ, tissue with tissue, almost cell with cell, established the connexion beyond doubt. Lankester would put the Trilobites in the same phylum, but in this we do not follow him. With regard to the brilliant but, to our mind, unconvincing speculations as to the connexion of some _Limulus_-like ancestor with the Vertebrates, we must refer the reader to the ingenious writings of Dr. Gaskell,[222] recently summarised in his volume on “The Origin of Vertebrates,” and to those of Dr. Patten in his article “On the Origin of Vertebrates from Arachnids.”[223] =Fossil Xiphosura.=[224] [Illustration: FIG. 159.—=A.=, _Hemiaspis limuloides_, Woodw., Upper Silurian, Leintwardine, Shropshire. Natural size. (After Woodward.) =B.=, _Prestwichia_ (_Euroöps_) _danae_ (Meek), Carboniferous, Illinois, × ⅔. (After Packard.) ]