595.2 97 



II. 



Are the Arthropoda a Natural Group ? 



IT was Von Siebold who, in 1845, formed the Insectaof Linnaeus, or 

 the Condylopoda of Latreille, into a primary division of the 

 Animal Kingdom, which he called Arthropoda. For thirty years this 

 was considered a natural group with close relationship between the 

 four classes which composed it, and in 1869 Fritz Miiller said that 

 there could scarcely be a doubt that all these classes were branches of 

 a common stem. The pedigree was traced as follows. The primitive 

 thysanuriform insect and the primitive myriopod were supposed to 

 have had a common hexapod terrestrial ancestor which was, probably, 

 a descendant of an aquatic zoea-form crustacean. The Crustacea and 

 the Arachnida were- traced to a primitive phyllopod, which appears 

 to have descended from an unsegmented nauplius. 



After the publication of Professor Moseley's papers on Pevipatits 

 capensis, the Malacopoda (De Blainville), or the Onychophora (Grube) 

 were taken out of the Vermes and admitted, by most zoologists, as a 

 fifth class of the Arthropoda. Professor Huxley, in his paper on the 

 Classification of Animals (1876), says that Peripatus is a low and primi- 

 tive arthropod, and thus affords evidence of the highest significance 

 as to the relations of the Annelida with the Arthropoda. But if 

 Peripatus is a primitive arthropod, it can only be a primitive tracheate 

 arthropod and not a primitive branchiate arthropod. To find the 

 common stock of Peripatus and the Crustacea, we must go back to the 

 earliest forms of worms ; and the characters which were formerly 

 supposed to shew a close relationship between the Crustacea and the 

 Insecta — including hollow-jointed limbs, exuviation of the skin, and 

 the structure of the compound eyes — cannot be due to inheritance, 

 but must have been evolved independently by what is called conver- 

 gence of characters. 



If this is correct, the Arthropoda must either be divided into two 

 primary divisions of the Animal Kingdom, each equal to the Mollusca; 

 or they must be united with the worms in Macleay's primary division 

 of Annulosa ; for I suppose it will be allowed that, in a natural classi- 

 fication, a group should include the common ancestors of each member. 

 If, however, the older conception of the origin of insects be correct, 

 then Peripatus cannot belong to the Arthropoda — for certainly it is 

 not a retrograde myriopod — but must be looked upon as a polychaete 

 worm specially modified for a terrestrial existence. 



