IN THE LAKV^ OF ANISOPTERID DRAGONFLIES. 189 



This is shown by two facts : — 



(a) The Undulate Type is found in the mature larvae of just those 

 verj genera which are universally acknowledged, by reason of the 

 number of other archaic characters which they possess, to be the most 

 primitive forms of Anisoptera still existing {Petalura, Cordulegaster, 

 Austrogomphus) . 



(?>) The Undulate Type also occurs in the young larvae of jEsclina and 

 Anax, which later on develop a Duplex System of gills of the Foliate 

 Type. 



2. The main longitudinal gill-folds in the Simplex System are homologous 

 with the so-called " rectal glands." This is also shown by two facts : — 



(a) Both occur as six longitudinal eversions of the rectal wall in the 

 positions 2, 4, 6, 8, 10, 12 on the clock-face. 



(h) The aborted main folds in the well-grown larva of ^scJma can be 

 followed back into the " rectal glands," of which they form the anterior 

 portion. 



3. The cross-folds in the Simplex System arise, ontogeneticaljy, later than 

 the main folds. They are therefore probably less archaic in origin than the 

 main folds. 



4. The Papillate Type of gill in the Simplex System is a remarkable 

 specialization from the more primitive Undulate Type. It may be regarded 

 as the highest expression of the Simplex System. 



5. The Duplex System arose from the more primitive Simplex System 

 (early Undidate Type) by gradual suppression of the main folds and by the 

 elevation of the function of the cross-folds to act as the principal gills of 

 the systen). This is shown by the two following facts : — 



(a) In the ontogenetic development of the Duplex System in JEsdma 

 and Anax a gradual suppression of the main folds and upgrowth of the 

 cross-folds leads to the development of the Foliate Type of gill from the 

 Undidate Type possessed by the young larva. 



{h) The separate gills of the bemibranchs of the Duplex System 

 correspond exactly, in their position and tracheal supply, with the cross- 

 folds in the Simplex System. 

 G. The fact that the middle line or axes of symmetry of the holobranchs 

 in the Dupjlex System occupy the positions 1, 3, 5, 7, 9, 11 on the clock-face, 

 cannot be used as an argument against the homology of the rectal gills with 

 the " rectal glands/^ as Sadones has used it. The change of position is 

 clearly explained by 5. 



7. The Implicate Type of the Duplex System is the most archaic type still 

 «xtant in that system. This is shown by : — 



(a) Its resemblance in details to the Undulate Typ)e. 

 (6) The very weak development of the basal pads. 



8. The Implicate Type does not, however, lie exactly along the direct 



