BY K. J. TILLYARD. 



69 



come off from the main stem usually at a much sharper angle, 

 especially along the narrower side of the gill-blade. 8uch nerve- 

 branches themselves branch once or twice externally, so that the 

 whole system of nerves, arising from the single branch, innervates 

 an area of the gill supplied by two, or even three, tracheal 

 branches from the main stem. As they extend outwards, these 

 branch-nerves finally divide up into single nerve-fibres, which 

 are easily followed out under a high power, in a freshly cut-off 

 transparent gill. Each single nerve-fibre is finally destined to 

 supply a small sensilla, in the form of a tactile hair or spine, 

 standing upon the edge of the gill. 



Besides the nerves shown in Text-fig.7, one can also detect 

 occasional single nerve-fibres running outwards from the main 

 nerve. There is also a series of stout, short branches, which 

 pass inwards from the main nerves to supply the central region, 

 or rachis, with its large tracheal trunks. Short single nerve- 

 fibres from these branches pass to the bases of the spines and 

 hairs of the midrib. 



As already indicated, in the median vertical lamellar gills of 

 the Lestid tyipe, all the branches arise from the two dorsal nerves, 

 the ventral nerves being apparently suppressed. 

 The Tracheal System. 



The whole tracheal system of the caudal gills is derived from 

 the dorsal tracheal trunks. Each of these divides into two in 



Text-fig. 8. 

 Diagram to show courses of main trachejtj at base of Triquetvo-quadiate 

 Gill-system (right lateral gill omitted). Lettering as on p. 109. 



the ninth abdominal segment. Of the two branches, the more 



