630 CAUDAL GILLS OF ZYGOPTERID LAFlViE, 



From this primitive type, the Triquetro quadrate Type de- 

 veloped, with four trachea? still present in the median gill, and 

 two in each lateral gill. Each of these numbers became reduced 

 to one-half in the evolution of the Lestid form of gill. 



From the Saccoid 'i'ype also, the Agrionid gill -forms were 

 developed, with two tracheae only in each of the three gills, both 

 in the Vertical and Horizontal Types; i.e., these lines of evolu" 

 tion only involved a reduction from four to two in the median 

 gill, the number in the lateral gills remaining the same as 

 before. 



(■\) The Intevnal Lamince and Alveolar Meshwork : — Vhefie 

 must be regarded as developments peculiar to the Odonata. 

 Probably both reached their maximum development in the spaci- 

 ous interior of the archaic Saccoid Type, with reductions or 

 restrictions in later types, culminating in almost complete loss 

 of both structures in the F.estid form of gill. 



These changes may be readily exhibited in the following 

 Table : — 



