BY H. J. TILLYARD. 



C15 



velopment consists only in the broadening of tlie blade of the 

 lamella. With this is correlated the formation of the rich 

 tracheal branchings into the blade, which are such a conspicuous 



larvie. 



feature of the gills in older 



Part iv. Phylogeny of thk Caudal Gills. 



In attempting to work out tlie Phylogeny of the Gills, it is 

 legitimate not only to seek for the evidence afforded by their 

 Ontogeny, but also to turn to the study of the homologous 

 organs in those Orders most closely related to the Odonata. 

 Since the Caudal Gills of Zygoptera are the modified cerci and 

 appendix dorsalis of the larva, and since the only other Orders 

 that can come into the question are the Perlaria and Plectoptera, 

 we must turn to the study of the caudal filaments oi their larva?. 



The Caudal Filaments of the Perlaria. (Text-fig.42). 

 The larv^ of the Stone-Flies or Perlaria possess only two 

 caudal filaments, viz., the cerci. These are, therefore, the homo- 

 logues of the two lateral gills of the Zygoptera, They are usually 



....... CU-......, 



hv- 



Text-fig.42. 

 T.S. through the two caudal filaments of a Mell-giown Perlid larva, sub- 

 family Leptoperl in(f, en, cuticle; dh, dorsal blood-canal; /iw, hsemo- 

 coele; hy, hj^podermis; nr, main nerve; tr, main tracliea; ( x 120). 



very long and many-jointed, but there are not lacking a number 

 of genera in which a great reduction in length, and also in the 



