ON THE ORIGIN OF VERTEBEATES FROM ARACHNIDS. 319 



of Scorpio and Limulus ; (4) that the remarkable fish-like 

 Pterichthys and related forms^ judging from their external 

 structure, are closely related to the Merostomata, and serve to 

 connect Arthropods with Vertebrates ; and (5) that the em- 

 bryology of Vertebrates in its main features can be reduced 

 to the Arthropod type. 



First let me state certain conclusions that have been reached 

 concerning segmentation in Arthropods. 



In Scolopendra each neuromere has four pairs of spinal 

 nerves ; the first two pairs in each neuromere are larger and 

 darker, and probably contain more sensory fibres than the two 

 following pairs. 



Certain facts indicate that this condition is the ground plan 

 of the nervous system in all Arthropods, and that the various 

 modifications of it found in other Arthropods are produced by 

 fusion of the nerves. The two sensory nerves tend to fuse 

 with each other first ; afterwards the two motor nerves ; and 

 finally the double motor and the double sensory nerves unite, 

 thus producing, in different groups of Arthropods, neuromeres 

 with four, three, two, and one pair of nerves. 



In Scolopendra the neuromeres appear to be double ; and, if 

 what we have indicated above is true, it follows that in all 

 Arthropods the neuromeres, and consequently the segments 

 themselves, are double. In support of this view we mention 

 the following facts : — (1) In all Arthropods carefully studied 

 two cross commissures have been found in each neuromere. 



(2) In Acilius the median furrow between these cross com- 

 missures is similar to that between the successive neuromeres. 



(3) In Acilius, according to my observations, there are two 

 pairs of tracheal invaginations in each segment : one pair, that 

 which is always readily seen, is situated near the anterior 

 edge of the segment ; the other, which is very rudimentary and 

 difficult to distinguish, is situated in the same line as the first, 

 but near the posterior edge of the segment. (4) In all the 

 insect embryos I have examined, and in almost all figures where 

 the tracheal openings were represented^ the stigmata were 

 situated near the anterior edge of the segment. (5) The 



