30 THOKELL & LINDSTRÖM, SILUKIAN SCORPION FROM GOTLAXD. 



are no doubt fully counterbalanced by the differences exhibited iu the structure of 

 the organs of resinration, and by the fact that the Scorpions and all the higher Arach- 

 nida are provided with Malpighian vessels, whereas the Merostoms (Limulus) and all 

 unquestionable Crustacea are devoid of these vessels. 



- Consequently, we cannot find in the organisation of the Merostoms, nor in their 

 development, sufficient reasons for removing them from the Crustacea and classing 

 theni with the Arachnida. Even if we assume that the Scorpions are directly descended 

 from the Merostoms, this is no ground for referring their water-breathing ances- 

 tors to the Arachnida: one might quite as reasonably refer the unknown, probably 

 Sauroid progenitors of the Birds, to the Class of Aves. The Merostoms would in fact 

 have been hrst changed into Arachnids when they had abandoned the water and be- 

 gun to breathe with trachece. 



But against tlie supposition of a direct descent of the Scorpions from the Me- 

 rostoms militates, among other things, the discovery of Palceophonus; for by this dis- 

 covery it has been demonstrated that the type of the Scorpions reaches nearly as far 

 back in time as that of the Merostoms. Ray Lankester, who thinks that the Scor- 

 pions have started from a Eurypterid with simple eyes, nearly similar to those of the 

 Scorpions, is obliged, in order to make this descent probable, to set forth the hj^po- 

 thesis that the »gill-books» of the Eurypterids have been gradually sunk or draAvn into 

 the body, and thus directly converted into the lameilar ti"acheffi or »lung-books» of 

 the Scorpions. Balfour^) again, who also inclines to the view that the Merostoms are 

 Arachnids, supposes that they are descended from some air-breathing animal, and that 

 they have secondarily acquired gills. But it would seem that none of these supposi- 

 tions is supported by what is known of the development of other animals. As to Bal- 

 four's hypothesis, it probably arose from his opinion that the air-breathing Arthro- 

 pods, the »Tracheates», form a natural unity coordinated with the »Branchiates»; but 

 this opinion can scarcely be maintained now that the closer relationship of the Arach- 

 nids with the Crustacea appears to be demonstrated. Packard^) and Bertkau^) have 

 already made several, as it seems to us well-founded remarks against the general assertion 

 that the Merostoms are Arachnids, as well as against Ray Lankester's above men- 

 tioned hypothesis. Bertkau shows, for example, the difficulty, by this hypothesis 

 of understanding the origin of the "spiracula cribriformia» of the Chelonethi and some 

 Opilions, which have just the same structure as the corresponding organs of the My- 

 riopoda, though they must have originated in an altogether different way if the said 

 hypothesis were true. 



According to Ray Lankester the Scorpions are the most ancient air-breathing 

 »Arachnids», and they have given rise to the entire series of existing forms of this 

 Class of animals, »to the Pedipalpi first, and through them to the Araneina, and through 

 the Araneina to the Acari». From this it would follow that the Arachnids have, in their 



') F. M. Balfodr, Comparative Embvyology, vol. I. p. 447. 



^) A. S. Packard jr., Is Limulus an Aracliuid ?, in Tlie American Naturalist, XVI, M 4. p. 287 et sequ., 



and in Ann. and Mag. of Nat. Hist., 5 Ser., IX, p. 396 et seqn. 

 •') Loe. cit., pp. 446 and 447. 



