No. 625] THE EVOLUTION OF ARTHROPODS 



,163 



madiilids, and tlieir immediate relatives, is quite closely 

 connected witli the Campodeoid group in the styli-bear- 

 ing subdivision of tlic Apterygota; but tlicir moutli parts 

 are ectognathous, and in their gonei-al organization they 

 approach remarkably closely to the lower Pterygota; 

 so that they may be said to occupy a position annectent 

 between the lower Pterygota and the Cani]X)dooid group. 

 The members of the Le})ismoid group seem to have in- 

 herited more characters from the crustacean side of tlu^ir 

 common ancestry than from the symphylan side, while 

 the members of the Proturan group seem to liave inher- 

 ited more characters from the symphylan side, and the 

 members of the Campodeoid group appear to i)artake to 

 some extent of characters occurring in botli the crusta- 

 cean (isopod) and symphylan sides of their common 

 ancestry. 



It miglit be possible to exi)lain tlie pre^eiice of both 

 crustacean (isopod) and s\unphylan characters in the iii- 

 sectan stem })y supposing that the ci'uslaccaiK insccian 

 and symphylan ''currents'' in the "onward flow ol' lilV." 



viduality a- 'tlieir ''waters'' oinoru,. r,,,ni the comnum 

 -ti-cani at their s.)uiro. ncN .■nlh'h— ha^r an iMtcnnin- 

 gliiig or connninu'ling o[' c.nitiunon- watri> a^ thry How 

 side by side, before ultiitiately <livt.'r<iinii- too gi'catly for 

 such an interminding. This idea, liowevei'. miglit in a 

 sense be interpreted as meaning tliat the Synii)hyla-like 

 insects were descended from Sympliyhi. and the Crus- 

 tacea-like insects from Crustacea (/. r.. isoi^od Crus- 

 tacea), whereas insects as a whole were jn-obahly not 



common ancestral source. Tlie forms composinu' this 

 common rnx't'stral source, however, differed amoiiir them- 

 selves ver\ ureatiy. although the amount of diveruvnce 

 was probably not too o-reat to tu'event theii' heinii- grouped 

 in a single ela-^ oi- ]io-^ih!\ even in a -inuie subclass 

 or order. In tliis anee-tral in>eelan u-rouji. there were 

 doubtless isopod lila^ insects which I'esembled the most 



