114 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



eighth segment itself, and not behind it, a narrow anterior median 

 part of this segment being incorporated into the posterior margin of 

 the prosoma, while lateral parts of it form the anterior lateral lobes 

 of the opisthosomatic carapace (fig. 47 D, VIII). Six following seg- 

 ments of the opisthosoma are marked by the six pairs of impressions 

 bordering the median elevation of the carapace, and by the six pairs 

 of marginal spines. The intrasegmental division of the body into 

 movable parts is not an anomalous condition ; it occurs between the 

 thorax and the abdomen of many insects, and is a necessary mechanical 

 adaptation resulting from the primarily intersegmental attachments 

 of the longitudinal muscles. 



The six pairs of dorsal impressions on the opisthosoma of Limulus 

 (fig. 47 D) and a pair of similar impressions on the posterior margin 

 of the prosomatic carapace (p) form internally (B) a double series 

 of tergal apodemes, the "entapophyses" of Benham (1885), of which 

 the larger first pair (VIIIAp) is on the prosoma, and the other six 

 pairs (IXAp-XIVAp) are on the opisthosoma. The tergosternal 

 muscles of the five gill-bearing segments are shown by Benham to have 

 their dorsal attachments {i2h-i2f) at the bases of the first five opistho- 

 somatic apodemes, while the corresponding muscles of the opercular 

 segment (VIII) arise at the bases of the corresponding prosomatic 

 apodemes (12a). On the other hand, while the "external branchial 

 muscles" of the gill segments have their dorsal attachments (20b- 

 2of) just laterad of the first five opisthosomatic apodemes, the corre- 

 sponding muscles of the operculum take their origins also on the 

 opisthosomatic shield, but more laterally on the anterior lateral lobes. 

 The muscle attachments, therefore, show that the dorsal part of the 

 eighth segment has been divided between the prosoma and the opistho- 

 soma, or, as Benham says, the first pair of tergal apophyses has been 

 transferred from the opisthosoma to the prosoma. The dorsal longi- 

 tudinal muscles between the prosoma and the opi-sthosoma of Limulus 

 have been condensed into a single large bundle of fibers, the "arthro- 

 tergal muscle" of Benham, and the attachments of this muscle (fig. 

 47 B, y8) have extended somewhat anteriorly and posteriorly on the 

 two body regions to acquire greater efficiency as a fiexor of the 

 opisthosoma. 



The innervation of the hinge region of the carapace gives the same 

 evidence of division within the eighth segment as that furnished by 

 the musculature. As shown by Patten and Redenbaugh (1900), the 

 nerves of the genital operculum proceed from the composite ventral 

 ganglion of the prosoma, while the corresponding somatic nerves 

 ("haemal nerves" of segment VIII) are distributed to the anterior 



