606 



ON THE MORPHOLOGY OF THE CAUDAL GILLS OF 

 THE LARV^ OF ZYGOPTERID DRAG0NFLTE8. 



Part iii. (Oxtooexy), axd Part iv. (Phylo^iexy). 



By R. J. TiLLYAKD, M.A., B.Sc, F.L.S., F E.S., Linneax 

 Macleay Fellow of the Society in Zoology 



(With fifteen Text-fij^ures). 



IXDEX. 



PAGE. 

 IXTRODnTlON 606 



Part iii. — ()nt<)«;eny. 



Second Instar ... ... ... ... ... ... ... ... 608 



Third Instai- «>10 



Fourth Instar 610 



Fifth Instar 611 



.Sixth Instar 613 



Seventli Instar 614 



Part iv. — Phylokeny. 



Caudal Fihmients of the Perlaria ... ... .. (Jlo 



Caudal Filaments of the Pleetoptera • <>!" 



Phjdogenetic Conclusions ... ... ' '» I <S 



Evolution of the Caudal (4ills within tlie Suborder Zygoptera . <>'20 



Evolution of the Blood-System .. <J'2S 



Evolution of tlie Nervous System ... ... .• <i'2y 



Evohition of the Tracheal System ... ... 629 



Evolution of the Inteiiial Lamin;e and Alveolar Meshwork ... 630 



1'ahle of Evolutionary Chanijjes ... .• ♦>31 



Note on the Problem of the ()rii;in of the Constricted or Two- 

 jointed (Till 63-2 



IXTKODUCTION. 



Ill dealing with the Ontogeny of the Caudal Gills, it was found 

 necessary to study, by means of sections, all instars of the grow- 

 ing larva up to the attainment of the complete internal form of 

 structure. As this was found not to be reached until the seventh 



