154 



THE AMEBIC AN NATURALIST 



[Vol. LIII 



as Phytoptus, etc., and the apparent resemblance of tlie 

 adults to eriopliyid mites, so that provisionally, at least, 

 the Lingnatulida may be regarded as strongly aberrant 

 mites, while the Tardigrada are probably not related to 

 them, but to the Myzostomida, as has been previously 

 discussed. 



Since the arachnoid path of evolution has led off in a 

 direction widely divergent from the path followed in the 

 development of the Insecta, it is very difficult to under- 

 standi how Thorell came to the conclusion that such 

 highly developed arachnids {i. e., those occupying a posi- 

 tion far along the divergent line of development) as the 

 SolifugsB are intimately related to insects. Further- 

 more, since the trilobite trend of development leads off 

 toward the merostomes and the divergent evolutionary 

 path of the arachnoid forms, it is necessary to search 

 further back than the trilobites for a group standing 

 more nearly in the direct line of development eventually 

 resulting in the evolution of the insectan type, and for 

 this purpose the study of the branchiopod representa- 

 tives of the Copepoda-Branchiopoda group is much more 

 valuable. 



The members of the Copepoda-Branchiopoda group 

 which seem to be the nearest to the stem forms at the 

 base of the line of descent which ultimately leads up to 

 the insect type of development are the Notostraca (Apo- 

 didse) and anostracan Branchiopoda, which are likewise 

 very closely related to the trilobites, so that certain an- 

 cestral features are to be found in the trilobites also, 

 having been inherited from their common forebears ; but. 

 as was stated above, the trend of triloljite development 

 is toward the production of the eurypterid and arachnoid 

 type of development, and therefore leads away from the 

 line of development which eventually results in the pro- 

 duction of the insect type. Walcott, 1912, nuices with 

 Bernard, 1892, in regarding the Apodid.-p .-i- aiiioim the 

 lowest representatives of the Arthropoda ( altlioiiiiii cer- 

 tain copepods are also extremely primitive) and suggests 



