66 



CAUDAL GILLS OF ZYGOPTERID LAIiV^, 



most posterior pair. The whole nerve-supply of the caudal gills 

 is supplied from these last. 



On entering the tenth segment, the tenth segment nerves lie 

 latero-ventrally on either side of the rectum. At about one- 

 third of the distance through the segment, each nerve divides 

 into two strong branches, one passing slightly more dorsad, the 

 other slightly more ventrad, and the former bending slightly 

 outwards away from the level of the original nerve. Of these 

 two branches, the ventral soon gives off a strong nerve to the 

 wall of the rectum, the poste7'ior rectal nerve (Text-fig.6, rn), and 

 nv.. ■nv' 



Text-fig.6. 

 Diagrams to show positions of principal nerve of the gills, A dorsal, B 

 lateral view. a, anus; Ah, abdomen; c, eercoid; dl, dorsal nerve of 

 left lateral gill; cW , ditto of right lateral gill; dm, left, and dm' right 

 dorsal nerve of median gill; leg, left, and leg' right caudal gill; meg, 

 median caudal gill; ni'i^, left, and w\q, right tenth segment nerve; 

 R, rectum; rn, posterior rectal nerves; d, ventral nerve of left lateral 

 gill; 1)1', ditto of right lateral gill; cm, left, and imi, right ventral 

 nerve of median gill. Reconstructed from series of transverse sections. 



then, running straight on, it enters the base of the lateral caudal 

 gill of its own side, forming the ventral nerve of the lateral gill 

 {vl, vV). This is a large nerve, lying very conspicuously upon 



