1897. ARE THE ARTHROPODA A NATURAL GROUP ? 113 



In conclusion, once one fairly faces the Arthropod question, the 

 " Phylum " seems to disintegrate into a number of separate groups. 

 Three assemblages seem quite natural, (i) the Crustacea, (2) the 

 Arachnida,and (3) the Tracheata Antennata (Insecta, Alyriopoda, and 

 very probably Pevipatiis). The relations between these are yet to be 

 determined, and the common ancestor is almost certainly to be sought 

 for among animals which are not arthropod in structure. 



Malcolm Laurie. 



St. Mungo's College, Glasgow. 



The question raised by Professor Hutton seems to be twofold : 

 (i) Are the Arthropoda mono- or poly-phyletic ? (2) If polyphyletic, 

 is it justifiable to speak of them comprehensively under one title ? 

 Touching the first question : it is generally admitted that the key to 

 arthropod morphology is to be sought amongst the chaetopod worms. 

 It is also admitted, I take it, that among the arthropods those which 

 stand nearest to the chaetopods are the Prototracheata [Pevipatiis), on 

 the one hand, and the Phyllopod Crustacea (Apus), on the other ; 

 also that Apus is the most primitive type of the great aquatic 

 branchiate division, the Crustacea ; and that Pevipatiis, if associated 

 with any classes at all, must be looked upon as most nearly related to 

 the ancestral type of the terrestrial tracheate division, comprising 

 insects, centipedes, and millipedes. 



This conclusion leads to the inquiry whether the Pevipatus-\\]i.e 

 ancestor is derivable from the Apus-Mke ancestor or vice vevsa ; or 

 whether the two are traceable to a common hypothetical form, 

 possessing the characteristic arthropod features, and thus standing 

 apart from the Chaetopoda. The answer that is given to these 

 questions will be the answer to Professor Hutton's query concerning 

 the plurality of origin of the Arthropoda. 



To my mind the answer must be in the negative in each case ; 

 whence it follows that we must go back as far as the Chaetopoda to 

 find a connecting link between the Tracheata and Branchiata. 

 Moreover, it appears to me that the same result will be reached if we 

 select any other crustacean instead of Apus as the primitive type, and 

 if we discard Pevipatiis as the ancestor of the Tracheata, and choose 

 some other form, real or imaginary, to take its place. 



It should be borne in mind, however, that there is no connecting 

 link either between Pevipatiis and the Chaetopoda, or between Apus 

 and the Chaetopoda. The two forms seem, in fact, to have travelled 

 from the chaetopod starting-point along parallel roads ; and being 

 furnished with the same material in the way of parapodia, not to 

 mention other organs, they have utilised them in the same way in 

 response to similar needs, converting those in the neighbourhood of 

 the head into feelers and jaws, and others farther back into more 

 perfect organs for progression. It thus becomes possible, owing to 

 the disappearance of the intermediate stages, to give a diagnosis of 



