618 CAUDAL GILLS OF ZYfJOPTEKID LARV.f., 



Cere i. — Comparing these with the cerci of Perlaria, we see 

 that both possess two main nerves and a single closed afferent 

 blood-canal. But, in the Plectoptera, the closed blood-canal is 

 ventral in position, as in the lateral caudal gills of Odonata, and 

 not dorsal, as in the Perlaria. The two main nerves also lie 

 in tiie same positions as in the Zygoptera, and not as in the 

 Perlaria. There is no main longitudinal trachea developed at 

 all in the cerci of the Plectoptera.* 



Appendix D o r s a 1 i s. — As this is absent in the Perlaria, 

 we must confine our comparison to the median caudal gill of the 

 Zygoptera. We then notice a remarkable similarity in the two 

 homologous organs. For the closed afferent blood-canal is dorsally 

 placed in both, and both possess four maiii nerves, tivo being latero- 

 dorsal, and tivo latero-ventral. The only dififerences are tliat 

 there is no main trachea in the appendix dorsalis of the Plec- 

 toptera, and no development of internal laminit or alveolar 

 mesh work. 



Phylogenetic Conclusions. 



The following conclusions may, T think, be safely diawn from 

 tlie combined evidence of the Ontogeny and the comparison with 

 the homologous organs of the Perlaria and Plectoptera. 



(1) The relationship between the Perlaria, Flectoptera, and 

 Odonata. 

 Firstly, as regards the Perlaria, there are some fundamental 

 differences in the arrangement of the internal structures of the 

 cerci, as compared with those of the Zygoptera. These are (a) 

 the position of the main nerves, and ib) the dorsal position of 

 the closed afferent blood-canal. I think that these differences 

 are great enough to make us conclude that the evidence from the 

 cerci only reinforces the opinion which I had previously stated,! 

 from a consideration of other morphological characters, viz., 



* This applies to Atalophlebia, the only genus available here for studj'. 

 I cannot say whether it is true for the whole Order or not. 



t "The Biology of Dragonflies." Cambridge University Press, 1917, 

 pp.5-6. 



