Sub-class 2. Embolobranchiata. Order (i.) Scorpionidea. Order (ii.) Pedipalpi. Order (iii.) Araneae. Order (iv.) Palpigradi. Order (v.) Solifugae. Order (vi.) Chernetidea (= Pseudoscorpiones). Order (vii.) Podogona. Order (viii.) Phalangidea (= Opiliones). Order (ix.) Acarina. APPENDICES (i.) Tardigrada. (ii.) Pentastomida. CHAPTER X ARACHNIDA (_CONTINUED_)—DELOBRANCHIATA = MEROSTOMATA—XIPHOSURA SUB-CLASS I.—DELOBRANCHIATA = MEROSTOMATA. =Order I. Xiphosura.=[206] In his recent classification of the Arachnida, Lankester[207] has grouped the Xiphosura or King-crabs with the extinct Eurypterids or Gigantostraca under the name of Delobranchiata, better known under the name Merostomata[208] of Dana. The chief character of this group, and one which differentiates it from all the animals placed together by Lankester in the group Embolobranchiata, is that they have gills patent and exposed. The Xiphosura are, in fact, with the exception of a few marine Mites, the only Arachnids which now live in the sea as did their allies the Eurypterids in Palaeozoic times. With a few fresh-water exceptions, all other Arachnids have taken to life on land, and with a change from water-breathing to air-breathing came a change in the respiratory system, the gills becoming “lung-books,” or possibly tracheae, or disappearing altogether. A few years ago Pocock re-classified the Xiphosura, and his classification will be found on pp. 276, 277. It will be noticed that in his classification the generic name _Limulus_ has disappeared. I have, however, retained it in this article, partly because I regard the name as so well established that every one knows what it denotes, and partly because in a group which contains confessedly very few species, differing _inter se_ comparatively slightly, it seems unnecessary to complicate matters with sub-families and new names. Looked at from above a _Limulus_ presents a horse-shoe-like outline, from the posterior end of which projects a long spine. It is often called in America the Horsefoot-crab, but its common or vulgar name is the King-crab. Across the middle of the body is a joint, and this joint separates the prosoma from the meso- and metasoma which are in King-crabs fused together. The prosoma comprises all the segments up to and including the segment which carries the chilaria;[209] the mesosoma begins with the segment bearing the genital pores, and ends with the last segment which bears appendages; the metasoma comprises all the segments posterior to the last segment which carries appendages. The prosoma corresponds with the “cephalothorax” of some authors, and the meso-_plus_ the metasoma are equivalent to their “abdomen.”