178 ALEXANDER PETEUNKEVITCH 



young. The cephalothorax of Limulus is therefore the result 

 of fusion of the original cephalothorax with the chilarial and 

 opercular somites, and the articulation between the carapace 

 and abdomen is in reality an articulation between the opercular 

 and first gill somite, or what corresponds to the division Une 

 between the second and third abdominal somites in Arachnida, 

 as exemplified by the genital and comb somites in the scorpion. 

 The division line between the last thoracic and first abdominal 

 (chilarial) tergites lies immediately in front of the attachment of 

 the heart, i.e., somewhat in front of the line passing through the 

 two lateral eyes. 



The history of this forward displacement and fusion of origin- 

 ally abdominal somites cannot be gleaned from a study of the 

 external segmentation of Limulus embryos. On the ventral 

 surface segmentation is clear, but on the dorsal the first visible 

 segment is already the first gill segment. Something similar 

 may be seen in the scorpion. Here, in the adult, the third 

 abdominal tergite corresponds to the first lung stemite and there- 

 fore in reaUty representing the fourth abdominal somite. The 

 second tergite, corresponding to the comb, represents the third 

 abdominal somite. But the first abdominal tergite is the result 

 of a fusion of the first and second tergites of the corresponding 

 embryonic somites. The external segmentation is clear in young 

 embryos on the ventral surface, and in quite young embryos 

 is at least indicated by the even segmentation of the nervous 

 system as seen in longitudinal sections through these stages. 

 But when segmentation appears in the shape of transverse de- 

 pressions on the dorsal surface, the first visible abdominal ter- 

 gite corresponds already with the same tergite of the adult and 

 is therefore already the result of fusion. It may be argued that 

 if in Limulus abdominal tergites fused with the carapace, the 

 same may have happened in the case of the missing first tergite 

 of the scorpion. But this interpretation meets with too many 

 objections. Of these perhaps the clearest is presented in the 

 case of solpugids in which the thorax is still externally segmented. 



In my monograph of Palaeozoic Arachnida ('13) I have pointed 

 out that the Xiphosura must have developed independently 



