No. 625] THE EVOLUTION OF ARTHROPODS 177 



tors giving rise to those Apterygota which are provided 

 with flagelliform terminal appendages. 



The members of the Symphyla-Pauropoda group have 

 retained many characters present in the ancestors of the 

 ''Myriopoda" and Insecta. The Chilopoda are an off- 

 shoot from this stock and do not stand quite as near the 

 direct line of development of the insectan type. 



The Apterygota are the nearest known representatives 

 of the ancestors of winged insects, and while the first 

 insects to be evolved possibly were of types resembling 

 both the proturan forms and the campodeoid forms (or 

 even the machiloid forms), the lepismid type approaches 

 as nearly as any known forms to the lowest representa- 

 tives of the Pterygota. 



The first winged insects resembled the lepismids in 

 many respects, and their nearest living representatives 

 are the ephemerids and Plecoptera. The Plecoptera and 

 the fossil Pala^odictyoptera stand at the base of the lines 

 of descent of the higher forms, and, since the line of de- 

 scent of the Plecoptera has accompanied those of the 

 higher forms for a longer distance, they are even more 

 iini)ortant tliaii tlic Pahrodictyoptera foi- a )>1i\ louciu'ti.- 

 study of the evohition of liiglior inseots. Mest hiulicr 

 forms cluster al)out the Plecoptera and XcurDjitcra a> 

 nuclei representing synthetic types of tlie greatest ini 

 portance, and both types are of consideral)Ie aiitit]uity. 

 although the Neuroptera were possibly ultimately dv^ 

 scended from forms not unlike the Plecoptera (and 

 ephemerids). 



It is quite improbable tliat injects or arthropods in 

 general (as well as tlie iiuire iiiclii>l\ e groups) are of a 

 polypbyletie origin. The anrc^icr^ of insects, for exam- 

 ple, were of soveral ty])v>, >..nie foenihliiig the anee^t<.r< 

 of iso]„),ls. wlrih' others resoinhlc-l the aneo>tei- of tin' 

 Symphyla, etc.. and the line> of deveh-i.nient of all thivo 

 extend for some distance n.le by <i.le hcfore eaeh he-iii^ 

 to diverge from the other-. Those in>eets rex-iiiMin- 

 Symphyla were not descended from syini^hylid foi-ehcat^ 

 nor were those insects which reseinhlc isopods doccnded 



