132 THE NERVOUS SYSTEM [CH. 



of segments 3-8, the position being close to the base of seg. 3, 

 and gradually moving a little backwards in each succeeding 

 segment, until in seg. 8 the large ganglion lies close to half-way 

 along that segment. 



As, in the larva, there seem to be always eight abdominal 

 ganglia, one situated in each of segments 1-8, it has long been 

 a puzzle to entomologists as to what happened to the missing 

 abdominal ganglion at metamorphosis. The problem was at once 

 solved when I dissected the archaic Petalura (fig. 54). For there, 

 very distinct, but attached posteriorly to the metathoracic ganglion 

 in the thorax, was the missing abdominal ganglion (ag^. It seems 

 quite clear, though not yet definitely observed, that this fusion 

 must take place either before or during metamorphosis, and is 

 correlated with (a) the decrease in the size of the first abdominal 

 segment to a narrow ring, and (6) the great elongation of the 

 abdomen of the imago as a whole. This stretching out of the 

 abdomen is also probably the cause of the other ganglia lying 

 so far forward in each segment; and, in particular, the cause of 

 the second ganglion moving up into the first segment. Stated 

 in terms of the numbered segments, the positions of the ganglia 

 appear highly irregular. But if we look at the figure of an actual 

 dissection (fig. 54) we shall see that it is the lengths of the segments 

 which are highly irregular, and that the tremendous stretching 

 undergone by the nerve-cords at metamorphosis has been so fairly 

 distributed, as to give approximately equal intervals between the 

 ganglia. It seems clear, too, that the first abdominal ganglion 

 moved forward in correlation with the whole forward movement 

 of the synthoracic sterna, when the thoracic obliquity of the 

 imago became so great that the nerve-cord could not otherwise 

 have stood the strain. 



The first seven abdominal ganglia in Petalura are all closely 

 similar in size and structure. Each gives off a pair of strong 

 branching nerves to the segmental muscles of its own segment. 

 Branches from these pass to the dorsal vessel. Smaller nerves 

 are given off to the muscles of the alimentary canal. The eighth 

 abdominal ganglion (ag B ) is much larger than any of the others, 

 and clearly represents the three completely fused ganglia of 

 segments 8-10. It gives off three strong pairs of nerves (w 8 -n 10 ) 



