150 MR. E. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



Nnmher of Capillaries in the Gill-hasket. 



It is not easy to estimate the number of capillaries in a papilla. They are 

 sufficiently numerous to give a striated appearance to the papilla ; but the 

 diameter of the latter is very narrow, and hence I have placed the minimum 

 number at 20 complete loops. There are about 100 papillae developed along 

 a single longitudinal fold in Hemigomphus. Hence we get a total of 

 6 X 100 X 20, or 12,000 complete loops for the six folds. Adding 50 per cent, 

 for the cross-folds, we obtain a total of 18,000 loops as a conservative 

 estimate. This is a great increase in the number estimated for Austro- 

 gomphus (10,800), especially as the larvse of Hemigomphus examined were 

 not full-grown and were of smaller size than those of Austrogomjyhus. 



Considering that it belongs to the Simplex Syatem, the papillate type must 

 be given a very high place in the development series for the remarkable 

 level of efficiency attained by it. Judging also by other attempts made in 

 the course of evolution to develop papillse on much more highly developed 

 gill-folds of the Duplex System, we must regard these devices as being 

 amongst the most efficaceous yet evolved for the extraction of oxygen from 

 the surrounding water. 



B. Duplex System. 



Definition. — The Duplex System of gills consists of six double holo- 

 branchs whose middle lines or axes of symmetry lie in the positions 1, 3, 5, 

 7, 9, 11 of the clock-face. Each holobranch consists of two hemibranchs 

 possessing a tracheal supply derived from a single series of primary eff'erents. 

 Each hemibranch consists of a series of separate gills arranged in a direction 

 more or less transverse to the middle axis of the holobranch, and correspondr 

 ing, in position and development, with the series of cross-folds already 

 mentioned in the Simplex System. Main longitudinal folds, of the type and 

 position defined in the Simplex System, are either completely absent, or, if 

 present, are aborted and do not carry tracheae. 



2'he Tracheal Supply. — Although the Duplex System seems at first sight 

 to be so much more complicated than the Simplex, yet its tracheal supply is 

 much more easily understood. For, with the elimination of the main longi- 

 tudinal folds, the Duplex System may be seen to resemble the cross-folds 

 alone of the Simplex System. Thus, then, each series of primary efferent 

 tracheae (approaching the gill-basket at the positions 1, 3, 5, 7, 9, 11 of the 

 clock-face, respectively) supplies a complete holobranch in a corresponding- 

 position. When a primary efferent bifurcates, one branch, or secondary 

 efferent, goes to one hemibranch, the other to the other. So, then, each of 

 the twelve hemibranchs is completely supplied by tracheae from a single 

 complete series of secondary efferents. 



To give an example : — The more dorsal, or left-hand hemibranch of the 



