30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



are more closely related to the Arachnida than to the Xiphosurida. 

 Aside from the adaptations to terrestrial life by the scorpions, such 

 as the substitution of " lung books " for gills and the correlated sup- 

 pression of the abdominal appendages or their incorporation in the 

 sternal plates, the principal structural difference between the two 

 groups might be reduced to the elimination of one gill-bearing segment 

 in the eurypterids. 



VI. ARACHNIDA 



The number of postoral body segments present in adult or em- 

 bryonic stages of Arachnida varies from a maximum of 19 in Scor- 

 pionida to a minimum of 13 in Araneida (not considering acarid 

 forms in which the body segmentation is obscure). However, since 

 the scorpions are said to have only 18 embryonic postoral neuromeres, 

 it may be true as Petrunkevitch (1922) suggests, that two of the 

 adult segments of the scorpion " tail " represent a single primitive 

 somite, and that the maximum arachnid segmentation, therefore, may 

 include only 18 postoral somites, as in Eurypterida, Malacostraca, and 

 Hexapoda. On the other hand, as already noted, the fact that the 

 last lung books of the scorpions and the last gills of Xiphosura are 

 on segment XIII , though the last gills of Eurypterida are said to be 

 on segment XII, is suggestive that the eurypterids have lost a segment 

 in the gill-bearing region of the body. A terminal spine, poison claw, 

 or flagellum, which is presumal)ly the telson, is not counted in the 

 above enumeration of segments. 



The body division into prosoma and opisthosoma in Arachnida is 

 between segments VI and VII, as in Eurypterida, but segment VII 

 is always reduced and is often suppressed, so that the apparent divi- 

 sion is usually between segments VI and VIII. A differentiation of 

 the opisthosoma into a wider anterior preabdomen and a taillike post- 

 abdomen is conspicuous in Scorpionida (fig. 10 D), Palpigradida, and 

 Pedipalpida (fig. 11 A). In the first two of these orders the division 

 lies between segments XI]' and XV, but in the third it is between 

 segments XV and XVI. In no case does the arachnid subdivision of 

 the opisthosoma correspond with the eurypterid division if the latter 

 is actually between segments XIII and XIV , as it appears to be (fig. 

 10 B, C), but, as above noted, if it is assumed that the eurypterids 

 have lost a segment in the preabdomen, the segmentation and body 

 division of the eurypterids and scorpions becomes the same. 



The Arachnida have a single median genital opening in each sex. 

 which (except in some of the Acarina) always pertains to the second 

 o]:)isthosomal segment, or segment VIII, and thus has the same seg- 



