186 THE RESPIRATORY SYSTEM [CH. 



3. Ontogeny and Phylogeny of the Types. 



All newly-hatched Anisopterid larvae have a gill-system closely 

 resembling the Undulate Simplex Type. This persists throughout 

 the larval life of the more archaic groups (Petalurinae, Cordule- 

 gastrinae and the genus Austrogomphus). In the Gomphinae the 

 Papillate Type develops by evaginations of the gill surface, 

 separated by constrictions. In the Aeschninae the larvae are 

 born with only the main-folds present. The cross-folds appear 

 as small buds close to the secondary efferents, at the third instar. 

 They increase in size, much resembling the Libellulid lamellae at 

 first, only smaller. Later they overgrow the main-folds, and the 

 latter lose their tracheae. The foliate type is established in Aeschna 

 by the seventh instar. In Anax the humps develop earlier, and 

 the papillae form as groups of tiny evaginations during the sixth 

 and seventh instars. In the Libellulidae the larvae are born with 

 six main-folds, each very short, and with only two main undulations. 

 These are supported by only one or two cross-folds on either side. 

 The cross-folds are already as large as at the fourth instar of 

 Aeschna ; and, except for a slight crinkling, are clearly destined 

 to become lamellae. At the next ecdysis, the main-folds lose 

 their pigment, and become non-functional ; the cross-folds become 

 definite lamellae. At the fourth instar, the main-folds are quite 

 gone; the cross-folds number from five to six in a hemibranch. 

 Lamellae continue to develop at each instar, until the full number 

 is attained. 



We may summarize this evidence as follows: The Undulate 

 Simplex Type is the basic form of gill. The Papillate Simplex 

 Type is evolved directly from it. The Implicate Duplex Type 

 (ontogeny not yet examined) appears to be another simple modifi- 

 cation in a different direction, the cross-folds alone forming the 

 gills. The Foliate Duplex Type is a higher development from the 

 Implicate Type, by constricting-off of the separate cross-folds. 

 This line culminates in Anax. 



The Lamellate Type, though necessarily classified with the 

 other Duplex Types, is a distinct line of development from the 

 very base of the series. It appears to have arisen at a time 

 when only a small region of the rectum had become specialized 



