SECT. XVI PERIPATUS AND THE TRACHEATA 285 



point, for In most other respects all Arthropodan legs 

 strongly resemble one another, and the presence of 

 chelae on a certain number of anterior limbs Is a com- 

 mon occurrence. Again, is there anything in Limulus, 

 or in any Crustacean, which resembles the two chitl- 

 nous hooked-claws at the ends of the legs of Scorpio, 

 which the latter possess in common with all other 

 Tracheata ? Nor do we find in the Scorpion idae any 

 special development of the sixth pair of limbs such as 

 we have shown to be characteristic not only of the 

 Apodidse but of the Trilobitse, the Xiphosuridae, and 

 the Eurypteridae, and which is especially marked 

 in the last, although this is claimed as a transition 

 form between the Xiphosuridae and the Arachnida. 



We do not, then, admit that very much weight can 

 be laid upon this agreement in number of segments 

 and in number and form of limbs. It certainly cannot 

 outweigh, for purposes of classification, the tracheae 

 and the Malpighian vessels, the presence of which in 

 the Scorplonidae and other Arachnida classes them 

 unmistakably with the Tracheata. 



Even if we admit the possibility of the concurrent 

 development of tracheae and Malpighian tubules for a 

 second time, the improbability of such an occurrence 

 is so great that we should require much stronger 

 evidence than any which has been adduced before we 

 could accept it. It is, further, very improbable that such 

 a highly specialised animal as a species of Eurypterus 

 should develop exactly the same respiratory and excre- 

 tory adaptations to a land life as the more generalised 

 Annelidan ancestor of the other Tracheata. 



