384 J.J). TothUl — The Ancestry of Insects. 



unlike the corresponding structures in Cliilopods or insects. 

 It seems therefore less of a mental effort to regard these 

 animals as an independent offshoot from Polychaete annelids 

 than to regard them as a direct bridge between Annelids and 

 Cliilopods. 



Returning to the Trilobites, practically all that was said 

 concerning the similarities between generalized trilobites and 

 Pterygogenea would apply with even greater force to Cliilo- 

 pods. Regional divisions of head and trunk, polypody, head 

 segmentation, antennae, eyes and cerci all suggest affinities. 



If in Cambrian times an adventurous species of the Meson- 

 acidae had left its marine surroundings to discover the terres- 

 trial world, it seems a fair guess that gills would have been 

 exchanged for lungs or tracheae. With no further change than 

 the loss of gills and acquisition of tracheae (a change actually 

 accomplished in the Isopoda and certain Arachnoidea) our bold 

 adventurer would now resemble in many fundamental respects 

 a generalized Chilopod. 



The lack of knowledge concerning the trilobite head and 

 the all-important nervous system preclude the possibility of 

 attaching more than suggestive value to a trilobite derivation 

 of the Cliilopods. "With the present lack of data, however, 

 this derivation seems at least as logical as any of the various 

 ones suggested. 



Conclusion. 



In conclusion a survey of the available data shows that the 

 Pterygogenea are closely related to Cliilopods and were quite 

 possibly derived from an ancient stock in which the maxilli- 

 pedes were not developed as jaws. Also that the Cliilopods 

 were in turn very likely, though by no means certainly, derived 

 as Handlirsch suggests, from ancient generalized trilobites. 



The probability of this derivation is increased by comparing 

 the manner of development in each group. In the trilobites 

 Barrande, Beecher, and others have shown that practically all 

 the segments were added after the egg stage ; in the Cliilopods, 

 Zograv, Voerboff, and Heymons have shown that most of the 

 specialization by addition of segments takes place during the 

 egg stage ; in the Hexapods numerous investigations have 

 shown the segments arise only during the egg stage. . This 

 brief series exemplifies one of the most fundamental changes 

 that has been undergone in the entire history of animals. 



From the etiological point of view also this series represents 

 a transition from predaceous marine animals, to predaceous 

 terrestrial wingless forms, to predaceous terrestrial winged 

 animals. 



