CHAPTER X 
ARACHNIDA (CON TIN UED)—DELOBRANCHIATA = MEROSTOMATA— 
XIPHOSURA 
SUB-CLASS I.—DELOBRANCHIATA = MEROSTOMATA. 
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., “ Recherches sur l’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. 8., ‘‘The 
Embryology of Limulus,’ Journ. Morph. vii. p. 35, and viii. p. 195, 1892-3. 
Kishinouye, ‘‘On the Development of Limulus longispina,” Journ. Coll. Sct. 
Japan, v., 1892, p. 58. Patten, W., and Redenbaugh, W. A., ‘‘ Studies on Limulus,” 
Journ. Morph. xvi., 1900, pp. 1, 91. 
2 Quart. J. Mier. Sci. xiviii., 1905, p. 165. 
3 unpés=a thigh. 
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