426 RECTAL GILLS IN LARViE OP ANISOPTERID DRAGONFLIES, 



within the epithelial syncytium, /airly close nj) to the cuticle 

 (Fig.5). 



The capillaries are collected into bundles at the base of the 

 gill, forming five or six, stouter, short tracheae. These unite 

 together to form the efferent trachea of the gill, termed a 

 secondary efferent{es). They are surrounded by the hypobranchial 

 tissue at the base of the gill. The secondary efferents from the 

 gills on one side of a double-row unite in pairs with those from 

 the gills of the other side, forming a single longitudinal set of 

 pi'imary efferents^ which enter one of the main longitudinal 

 trunks of the body. 



We can now attack our problem directly. There are clearly 

 eight separate elements which go to form a single gill, viz., 

 (1) the cuticle, (2) the epithelial syncytium, (3) the pigment, 

 (4) the basal pad, (5) the hypobranchial tissue, (6) the blood- 

 plasma in the narrow blood-spaces, (7) the capillaries, and (8) the 

 larger tracheae and efferents. We have to determine which of 

 these are the essential elements in respiration, and which are 

 merely accessory thereto. 



We must class as accessories all those structures which are not 

 present in all types of rectal gill. For if, in certain cases, the 

 gill respires satisfactorily without them, it is clear that they 

 have a function only accessory to respiration in those gills in 

 which they are present. 



It is also clear that the part of the gill, which is concerned 

 with the extraction of oxygen, is the part which projects into the 

 water in the rectal cavity. Structures which are present only 

 at the base of the gill will, in general, therefore, be only acces- 

 sories. 



Thus we arrive at the following list of accessory structures: — 



(1). The pigment. — Although developed all over the lamella in 

 Hemicordulia, yet, in most forms, the pigment is confined to the 

 basal part of the gill. In many genera (e.g., Austrogorajjhus, 

 Petalura, G ordulegaster, De^idroceschna, Austrophlebia, Orthetrum) 

 no pigment is developed in any part of the gill. Hence we may 

 safely conclude that the pigment is purely an accessory element, 

 so far as respiration is concerned. 



