340 Proceedings of the Royal Irish Acade7ny. 



primitive Araclmids "were confined to the "hind- body, as has heen ahly 

 maintained by "Wagner ('95). 



But whatever may be the truth as regards this point, there can be 

 no reasonable doubt that in their free thoracic segments, the Solifugida 

 and the Palpigradi (Hansen and Sorensen, '97) retain a primitiTe 

 character. This then must be taken into account, together with the 

 indications furnished by the abdominal segmentation of the Scorpions 

 and Pedipalpi. When we come to speculate on the nature of the 

 primeyal Arachnids, we are clearly led, in this way, to imagine an 

 Arthropod with a head carrying three pairs of limbs, whereof the fore- 

 most were three-segmented chelicerse, a thorax with three free segments, 

 each with a pair of limbs, and an abdomen of thirteen segments — the 

 foremost the pre-genital or waist-segment, detected in the embryo 

 scorpion by Brauer ('95), and in the embryo of Spiders by many 

 observers, represented by the metastoma of Eurypterids, and perhaps 

 by the sternum of the Scorpions. Each abdominal segment from the 

 second to the seventh, inclusive, had a pair of appendages canying 

 gill-plates. Of these the two foremost pairs are represented in the 

 Spiders, the four hindmost in the Scorpions. There was, of course, no 

 specialized " post-abdomen," as in Scorpions ; but the segments tapered 

 gradually towards the tail-end, the hindmost beaiing some kind of 

 telson. This conception of the ancestral Ai-achnid agrees closely with 

 that figured by Pocock ('93b), except that he supposes a completely 

 fused cephalothorax, thereby allowing no weight to the evidence of 

 the free thoracic segments in the SoKfugida and the Palpigradi. 



Having arrived at this result, we are now in a position to inquire 

 whether the " Gigantostraca" — the Limuloids and Eurypterids — should 

 be considered as belonging to the Arachnid class. That Limulus is 

 nearly related to the Merostomata, and can probably be derived from 

 the same ancestry as that order, through such forms as the Silurian 

 Bunodes and Hemiaspis, and the Carboniferous Belinurus and 

 Prestwichia, can hardly admit of doubt. And it is further evident 

 that Eurypterus and the Limuloids could, like a Scoi-pion or a 

 Galeodes, be derived from the ancestral Arachnid that we have just 

 imagined. The correspondence in the segmentation of Eurypterus and 

 the Scorpion is so close and remarkable, that we are forced to admit 

 an afl5.nity. This point has been sufficiently argued by Lankester and 

 others ; but to claim that both the ^Merostomata and Xiphosura ought 

 to be classed as Arachnida does not involve the belief that Scorpions 

 are descended from Eurypterids, still less from King-crabs I In view 

 of the specialized fusion of both cephalo-thoracic and abdominal 



