572 E. RAY LANKESTER. 



of such a class as the Crustacea rather thau a range from 

 lower more generalised to higher more specialised forms such 

 as that group and also the Arachnida present. It seems to 

 be a legitimate conclusion that the most primitive Hexapoda 

 were provided with wings, and that the term Pterygota 

 might be used as a synonym of Hexapoda. Many Hexapoda 

 have lost either one pair or both pairs of Avings ; cases are 

 common of wingless genera allied to ordinary Pterygote 

 genera. Some Hexapods which are very primitive in other 

 respects happen to be also apterous^ but this cannot be held 

 to prove that the possession of wings is not a primitive 

 character of Hexapods (compare the case of the Struthious 

 birds). The wings of Hexapoda are lateral expansions of the 

 terga of the second and third thoracic somites. They appear 

 to be serial equivalents (homogeneous meromes) of the tracheal 

 gills, which develop in a like position on the abdominal 

 segments of some aquatic Hexapods. 



The Hexapoda are all provided with a highly developed 

 tracheal system, which presents considerable variation in 

 regard to its stigmata or oritices of communication with the 

 exterior, in some a serial arrangement of stigmata compar- 

 able to that observed in Chilopoda is found. In other cases 

 (some larvae) stigmata are absent; in other cases again a 

 single stigma is developed, as in the smaller Arachnida and 

 Chilopoda, in the median dorsal line or other unexpected 

 position. When the facile tendency of Arthropoda to develop 

 tracheal air-tubes is admitted, it becomes probable that the 

 trachea) of Hexapods do not all belong to one original system, 

 but may be accounted for by new developments within the 

 group. Whether the primitive tracheal system of Hexapoda 

 was a closed one or open by serial stigmata in every somite 

 remains at present doubtful, but the intimate relation of the 

 system to the wings and tracheal gills cannot be overlooked. 



The lateral eyes of Hexapoda, like those of Crustacea, 

 belong to the most specialised type of " compound eye," 

 found only in these two classes. Simple munomeniscous eyes 

 are also present in many Hexapods. 



