178 THE RESPIRATORY SYSTEM [CH. IX 



C. The Branchial Basket. 



This unique, highly efficient, and very beautiful structure is 

 only found in the larvae of Anisoptera. The "basket" is formed 

 by expansion of the anterior two-thirds or more of the rectum, 

 which becomes a wide barrel-like chamber. From its walls the 

 gills project inwards in six longitudinal sets, developed originally, 

 as I have recently shewn [179], from the six rectal pads, which 

 remain undifferentiated in the posterior part of the rectum. The 

 whole structure of the branchial basket is so complicated, that 

 we shall divide our account into five sections: (1) the efferent 

 tracheal system, (2) the types of gill-system, (3) the ontogeny 

 and phylogeny of the types, (4) the histology of the gills, and 

 (5) the physiology of respiration. 



1. The Efferent Tracheal System (fig. 78). 



Originally developed as a series of branches from the main 

 trunks to the rectum, this tracheal system, on the birth of the 

 larva, at once takes on a definite efferent function, conveying the 

 gas received by the gills from the water to the main longitudinal 

 trunks. Thus, while it is most convenient to trace the system 

 inwards from the large trunks to the fine capillaries, we shall 

 use an efferent notation in describing them. The arrangement 

 may be gathered from figs. 76, 78. 



There are six series of principal branches, or primary efferents 

 (efj), to the gill-basket, arranged in three pairs. Two series come 

 off from the dorsal trunk of each side, one from the visceral trunk. 

 The ventral trunks have no part in the formation. Each primary 

 efferent, on approaching the gill-basket, divides into two secondary 

 efferents (ef%), which pass through the muscular tunic to the bases 

 of the gills. Here they break up in various manners, according 

 to the type of gill. These branches enter the gill-base, and give 

 off a very large number of tracheal capillaries, fine tubes without 

 any spiral thread, destined to receive the gas from the water in 

 the rectum. Each capillary forms a complete loop along the free 

 border of the gill, finally returning to the same secondary efferent 

 from which it originally arose. The different systems of branching, 

 and the courses taken by the capillaries, will be best understood 

 by reference to fig. 80. 



