Appendages in Hexapoda. 205 



dermic elements, but which possesses neither muscles, nerves, 

 nor tracheae. With the thirtieth day commences the retro- 

 gression of the abdominal sacs, and " on the excluded embryo 

 we can find only the closed-up scar of its peduncle." 



Conditions like those of the embryonic development of the 

 insects just referred to are to be found persistent in the mature 

 representatives of a section of Hexapoda, which, although 

 nearly related to the Orthoptera, has been justly separated by 

 F. Brauer from the other insects as " Apterygogenea," 

 which never have possessed wings. 



Thus in Campodea, the genus of Thysanura which in 

 general stands nearest to the primitive form, there are leg- 

 like appendages upon the first abdominal segment, and these 

 in young animals are comparatively more strongly developed 

 than in the adults ; at the same time the whole ventral surface 

 of this segment, by the abundance of cells and staining 

 faculty, reminds one of embryonic tissue. The appendages 

 are seated in the same direction as the thoracic legs, and also 

 show an indistinct articulation into two or three joints. Thus 

 only the portion of the ventral plate which is situated between 

 them is to be regarded as the " ventral shield." The aborted 

 musculature of these leg-rudiments, which completely resemble 

 the developing extremities of the Symphyla, is also to be 

 deduced from that of the thoracic legs, and in their segmental 

 division, such as appears characteristic of mesoblastic appen- 

 dages, it is traceable to the last joint of the rudiment. On 

 the next (second) abdominal segment instead of the leg-like 

 appendage there appears externally a cheliform movable 

 piece, and within a cutaneous sac lined with very large hypo- 

 dermic cells in part glandularly developed, which is protruded 

 by the inflow of blood and retracted by special longitudinal 

 cutaneous muscles attached to it at the apex. Towards the 

 end of the body, at least to the extremity of the seventh abdo- 

 minal segment, the planing down of the duplicatures and their 

 fusion with the ventral shields gradually becomes more and 

 more intense, at the same time the cutaneous sacs decrease in 

 size, and the cheliform spur increases, so that even on this 

 account the former may be claimed as older formations. At 

 the eighth abdominal segment the saccules return within the 

 body and at the same time come together in the middle in 

 front of the opening of the sexual organs ; as in Japyx, the 

 movable abdominal spurs are wanting from this segment 

 onwards also in Campodea. 



In the largest representative of the Thysanura, Japyx 

 giyas, there is on each side of the narrow, unpaired ventral 

 shield of the first abdominal segment a tripartite mass of 



