180 THE RESPIRATORY SYSTEM [CH. 



2. The Types of Rectal Gill-System (figs. 78-80). 



The gill-system may be either Simplex or Duplex. Distinct 

 types of gill are found in both systems, and have been named 

 according to their form [138, 179]. 



The Simplex System (figs. 78-79 A, 80, i). 



In the Simplex System there are six principal longitudinal 

 gill-folds (the main-folds, mf] running the whole length of the 

 basket, and arranged in the positions denoted by the even numbers 

 on a clock-face diagram, i.e. in the same positions as the rectal pads 

 of the imago. These are supported, to right and left alternately, 

 by a double series of cross-folds (cf). A complete main-fold with 

 its cross-folds is called a holobranch. The twelve sets of secondary 

 efferent tracheae correspond with the twelve sets of cross-folds. 

 Each, however, after supplying a branch to its cross-fold, continues 

 on to enter the main-fold. Thus each main-fold receives a double 

 tracheal supply, from right and left alternately. We see, from 

 fig. 78 A, that each main-fold, therefore, is in connection, by 

 alternate tracheae, with two sets of primary efferents. Or, in 

 other words, the two sets of secondary efferents, arising from 

 one series of primary efferents, do not enter a single main-fold, 

 but pass, one to the left side of one main-fold, the other to the 

 right side of the next main-fold. This complicated arrangement 

 becomes much simplified in the Duplex System. 



The main-folds are crinkled and grooved, projecting obliquely 

 backwards into the rectum in the manner seen in fig. 79 A. The 

 cross-folds are also crinkled, but do not project so far inwards. 



There are two very distinct types of Simplex System known : 



(1) The Undulate Type (figs. 78-79 A, fig. 80, i). This occurs 

 in the Cordulegastrinae,Petalurinae and in the genus Austrogomphus. 

 The gill-folds remain entire, and are undulated along their whole 

 length. The free edge of the gill carries minute spines at short 

 intervals. The tracheal branches approach fairly near to the free 

 edge, giving off capillaries along their whole length. These 

 capillaries run nearly straight to the free border, and then turn 

 round to form loops, returning to a near-by branch of the same 

 secondary efferent. Pigment is seldom developed in these gills. 



