NO, 14 INSECT ABDOMEN^ — SNODGRASS 25 



to the trilobites. It is important to note, however, that in forms with 

 six pairs of legs the body is divided into prosoma and opisthosoma 

 between segments VIII and IX as in Xiphosurida, whereas in the 

 other Chelicerata the division is between segments VI and VII. 



IV. XIPHOSURIDA 



The body of a horseshoe " crab " is distinctly divided into an 

 anterior section and a posterior section, commonly called the prosojna 

 and the opisthosoma (or cephalothorax and abdomen) ; but, as will 

 presently be shown, the dividing line between the two parts does not 

 correspond with that separating the two body regions termed prosoma 

 and opisthosoma in the Arachnida, whereas, on the other hand, it is 

 identical with that between the two body regions of Pycnogonida hav- 

 ing eight pairs of appendages. 



Each section of the xiphosurid body is covered by a broad carapace 

 having the marginal parts widely inflected on the ventral surface of 

 the animal. The opisthosoma is freely movable on the prosoma by 

 a long transverse hinge between its carapace and the prosomal cara- 

 pace. On the ventral surface (fig, 8 A) are 13 pairs of appendages, 

 including the chelicerae (Chi), the pedipalps {Pdp), four pairs of 

 legs {1L-4L), the chilaria {Chi), and 6 pairs of broad platelike 

 structures overlapping each other posteriorly against the under sur- 

 face of the opisthosoma. The first pair of these lamelliform appen- 

 dages are united to form a large flat operculum (Opl), which mostly 

 covers the following five appendages {Apds IX-XIII). On the pos- 

 terior (dorsal) surface of the operculum are located a pair of small 

 papillae containing the genital apertures in each sex (fig. 9 B, Fen). 

 The other opisthosomal appendages bear each a group of lamelliform 

 gills {D,Brn). 



The dividing line between the two parts of the body in Xiphosura 

 appears at first sight to lie between segments VII and VIII, since the 

 chilaria clearly belong to the prosoma and the operculum is associated 

 with the opisthosomal gill plates. A closer inspection, however, shows 

 that the first appendages actually carried by the opisthosoma are the 

 first pair of gill— bearing plates, and that the operculum hangs from 

 a membranous area of the venter that is continuous laterally with the 

 posterior angles of the prosomal carapace. Hence, as shown by 

 Petrunkevitch (1922), the true division between prosoma and opis- 

 thosoma in Xiphosura lies between the somite of the operculum 

 (segment VIII) and that of the first gill-bearing appendages (seg- 

 ment IX). The body division in the Xiphosurida, therefore, cor- 



