68 



CAUDAL GILLS OF ZVGOPTERID LARV^, 



courses appear to be far more irregular, and their branchings 

 more numerous and more difficult to follow, than in the othei* 

 types. If we examine a cross-section of one of the main nerves 

 lying upon the wall of the blood-canal, in any gill-type, we shall 

 see that it may either project well into the hsemocoele, or it may 

 lie almost flush with the wall, or it may be closed off and lie 

 just outside the canal. One need not attach any importance to 



variations in this respect, 

 since they would appear to 

 be mainly due to the con- 

 dition of the blood -canal 

 (and, indeed, of the whole 

 gill) at the moment of fix- 

 ation. For if the blood- 

 canal be distended, the 

 nerves will appear as if 

 projecting into it, whereas 

 if it be collapsed, they may 

 appear to be outside its 

 wall . The important point 

 to notice is that, in any 

 case, the nerve runs in a 

 small iierve-canal oi itsown, 

 as shown in Plate vi., figs. 

 44, 45. The wall of this 

 canal is similar in struc- 

 ture to the wall of the 

 blood -canal. 



By referring to Text-fig. 



nv \\m tr lliivnm 



Text-tig. 7.* 



7, we may follow the 

 courses of tlie main nerves 

 and their principal branches throughout the gill. The nerve- 

 branches, in the case of a vertical lamellar gill of the Ayrioind 

 type, are much less numerous than the tracheal branches, and 



■■' Portion of stained Avhole mount of gill of AvMroayrion ajane (Selj's), 

 to show hypodenn-nuclei {nn), blood-canals {km), main nerves [nr) and 

 their branches, internal lamina' (//) and main trachea^ [tr), Mith their 

 branches:Tf xlOO). 



