1897. ARE THE ARTHROPODA A NATURAL GROUP P 115 



four classes, Crustacea, Insecta, Myriopoda and Arachnida, are all 

 branches of a common stem, is given in the long and interesting 

 foot-note which he contributed to Dallas's translation of his " Fiir 

 Darwin " in 1869. If this opinion be correct, "it is evident," he says, 

 " that the water-inhabiting and water-breathing Crustacea must be 

 regarded as the original stem from which the other terrestrial classes, 

 with their tracheal respiration, have branched off." The tracing of 

 the pedigree must be sought elsewhere, for in the conclusion of " Facts 

 for Darwin," after glancing at a suggested connection between the 

 Rotatoria and Crustacea, Fritz Miiller says, " But I can see nothing 

 certain. Even towards the nearer provinces of the Myriopoda and 

 Arachnida I can find no bridge. For the Insecta alone, the develop- 

 ment of the Malacostraca may perhaps present a point of union." 



It is obvious that if the primitive arthropod is to be regarded as 

 anything like a crustacean nauplius, Peripatus cannot be a primitive 

 arthropod. Still it might be an arthropod, without being a primitive 

 one. Primitive and tracheate seem to be scarcely compatible terms. 

 Among the Crustacea some of the terrestrial Isopoda have appendages 

 furnished with tracheae. It would be very difficult to believe that in 

 the Isopoda such a character was primitive and not a late acquisition. 

 Considering the remoteness of the geological time at which we find 

 the classes of Arthropoda already distinct from one another, there are 

 but dim hopes of our ever positively discovering the point at which 

 their lines of ancestry meet. The "probable antiquity" which Mr. 

 R. I. Pocock {yoiirn. Linn. Soc. Zool. vol. xxiv., p. 518, 1894) attributes 

 to Peripatus, Peripatoides, and Peripatopsis may be indefinitely great, but 

 hitherto it has told us nothing of the form from which they started on 

 their peripatoidal career. We cannot tell how many stages have 

 been made in advance, nor how many backward. Although this 

 anomalous group has only been seriously studied for a short time, 

 there are already three names for it at our disposal, Malacopoda, 

 Onychophora, and Prototracheata, of which the last seems the least 

 suitable because it implies a yet unproved hypothesis. But there is 

 surely no urgent necessity to decide whether the group belongs to 

 one or other of the primary divisions of the animal kingdom as 

 hitherto accepted. Whether it be left standing on the outskirts of 

 the Vermes or of the Arthropoda, in the modern activity of research 

 there is always a chance that fresh light will be thrown on its affinities, 

 and its position then assigned on reasonable grounds, and not by 

 premature guessing. 



T. R. R. Stebbing. 

 Tunbridge Wells. 



Summary. 



Though the opinions as to the constitution of the Arthropoda, 

 advanced by the specialists who have kindly replied for us to Captain 

 Hutton, are as diverse as possible upon many points of taxonomy, 



