CHAPTER X 



AKACHNIDA {CONTINUED) DELOBRANCHIATA = MEROSTOMATA 



XIPHOSUKA 



SUB-CLASS I.— DELOBEANCHIATA = MEEOSTOMATA 



Order I. Xiphosnra.^ 



In his recent classification of the Arachnida, Lankester^ has 

 grouped the Xiphosura or King-crabs with the extinct Euryp- 

 terids or Gigantostraca under the name of Delobranchiata, better 

 known under the name Merostomata ^ of Dana. The chief 

 character of this group, and one which differentiates it from all 

 the animals placed together by Lankester in the group Embolo- 

 branchiata, 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. 



' Woodward, "On some Points in the Sti'ucture of the Xiphosura, having 

 reference to their relationship with the Eurypteridae," Quart. J. Geol. Soc. xxiii., 

 1867, p. 28, and xxviii., 1871, p. 46. Milne Edwards, A., "Recherches sur I'anat. 

 des Limules," Ann. Sci. Nat. (5), xvii., 1873, Art. 4. Lankester, E. R., " Limulus 

 an Arachnid," Quart. J. Micr. Sci. xxi., 1881, p. 504. Kingsley, J. S., "The 

 Embryology of Limulus," Journ. Morph. vii. p. 35, and viii. p. 195, 1892-3, 

 Kishinouye, "On the Development of Limulus longispina," Journ. Coll. Sci. 

 Japan, v., 1892, p. 53. Patten, W., and Redenbaugh, W. A., " Studies on Limulus," 

 Journ. 3Iorph. xvi., 1900, pp. 1, 91. 



" Quart. J. Micr. Sci. xlviii., 1905, p. 165. 



^ fxrjp6s = a, thigh. 



259 



