ON THE ORIGIN OF VEETEBRATES FROM ARACHNIDS. 331 



or all of these haemal nerves in both Scorpio and Petromyzon 

 supply segmental respiratory organs. (5) One large nerve in 

 the vagus group of both Scorpions and Vertebrates is con- 

 spicuous on account of its sensory nature and lateral position. 

 (6) In Scorpio the neural vagus nerves supply a specially 

 modified appendage called the comb. There is reason to sup- 

 pose that originally the vagus of Vertebrates also supplied a 

 specially modified appendage — the pectoral fin. The resem- 

 blance between these appendages will be considered later. (7) 

 In Scorpion the neural roots of the vagus and their ganglia 

 decrease in size from before backwards ; this is remarkable, 

 since we should naturally expect the third or comb root to be 

 the largest. Van Wyhe has discovered a similar condition in 

 the '^anlage'^ of the vagus of Selachians. (8) The vagus nerves 

 of Vertebrates and Scorpions are derived from four neuro- 

 meres not belonging originally to the brain, which are more 

 intimately fused with one another than are those in front 

 of or behind them, while the nerves themselves are the most 

 complex and most modified nerves in the whole body. This 

 condition is all the more extraordinary since, from their posi- 

 tion, we should naturally expect these nerves and neuromeres 

 to be intermediate in character between those of the head and 

 trunk. Finally, (9) if we count three segments to the fore- 

 brain, the vagus neuromeres in Scorpio and Vertebrates fall 

 in exactly the same place in the series — that is, in the tenth to 

 thirteenth segments inclusive. Nowhere else in the animal 

 kingdom do we find four segments in the middle of the body 

 with these extraordinary characters. 



There are, of course, important difi'erences between the 

 vagus of Scorpio and that of the lowest Vertebrates; but we 

 are content to show here that there are very decided resem- 

 blances between them, and that the differences are not greater 

 than those found among the Vertebrates themselves. It is 

 evident that a still further modification of the vagus of Scor- 

 pio, in the direction along which it has already advanced so 

 far, would lead up naturally to the most primitive condition of 

 the vagus in Vertebrates. 



