CHAPTER X 



AKACHNIDA {CONTINUED) DELOBRANCHIATA = MEKOSTOMATA 



XIPHOSUKA 



SUB-CLASS I. — DELOBEANOHIATA = MEEOSTOMATA. 



Order I. Xiphosura.^ 



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. 



1 Woodward, "On some Points in the Structure 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., "Reoherches 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 iimMtos, " 

 Journ. Morph. xvi., 1900, pp. 1, 91. 



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



' /j.rip6s = a, thigh. 



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