IN THE LARViE OF ANISOPTERID DRAGONFLIES. 139 



It will be noticed that a single type of gill, and that the most highly 

 specialized of all, characterizes the whole of the Libellulidse, while the other 

 four types are distributed in a definite phylogenetic sequence within the 

 .SDschnidse. 



G-ENBRAL Study of the Branchial Basket. 



A. Its Position luitliin the Abdomen. 



Longitudinal Extent. — When contracted the branchial basket occupies the 

 whole length of segments 7 and 8, together with a small part of the posterior 

 end of 6 and of the anterior end of 9. When expanded the gill-folds can be 

 seen to reach forward as far as the middle of 6^ or even a little further, and 

 backwards nearly to the posterior end of 9. In length the gill-basket 

 occupies three-fourths or more of the whole extent of the rectal region. 



Position in Transverse Section (text-fig. 1). — The gill-basket, as seen in 

 transverse sections of the abdomen^ occupies a large, roughly circular space 

 in the middle^ surrounded by the hsemocoele. Dorsal to it lies the heart (lit), 

 and immediately ventral to it is the ventral nerve-cord [vg). On either 

 side it is flanked by the fat-body (/6) and a mass of tracheae. Of these, the 

 two large dorsal trunks {dt) lie dorso-laterally on either side, close to the 

 gill-basket, while the smaller visceral trunks (yst) lie similarly ventro-laterally. 

 Both dorsal and visceral trunks give off numerous branches^ which enter the 

 gill-basket. Each dorsal trunk gives off two sets of branches, each visceral 

 trunk only one. The arrangement of these branches is discussed below 

 (p. 181). The ventral trunks (ynt^ lie at about the same level as the visceral 

 trunks, but further apart, and send no branches to the gill-basket. 



In the hgemoccele, between dorsal and visceral trunks, there may usuallj be 

 seen two or three Malpighian tubules (inlp) running backwards from their 

 point of origin at the anterior end of the proctodseal part of the intestine. 

 Most of these tubules, however, lie well forward from the level of the 

 gill-basket. 



The other structures to be noted in a cross-section through the abdomen at 

 this level are the large masses of the segmental muscles. These consist of 

 dorsal or tergal longitudinal muscles {mid), ventral or sternal longitudinal 

 muscles (inlv), and the smaller masses of the dorso-ventral or tergo-sternal 

 transverse muscles {mt) occupying the angular spaces near the junction of 

 the tergite with the sternite. 



B. Its Supports. 



The gill-basketj being part of the continuous alimentary tube, has no 

 special supports of its own, unless its weak muscular tunic can be considered 

 as such. But, just posterior to the gill-basket, and, in the later larval stages, 

 also connected with the narrower posterior end of the basket itself, there can 



