MORPHOLOGY OF BEAIN AND SENSE ORGANS OP LTMULUS. 85 



Limulus and Vertebrates unless they are genetically re- 

 lated, 



VI. Comparison of other Brain Regions of Limulus 



WITH THOSE of VERTEBRATES. 



In any near relative of Limulus there might be from five to 

 ten pairs of supra-branchial nerves, according as the cheli- 

 cerse and the vagus segments retained their sense organs. In 

 Limulus there are five pairs, those belonging to the post- oral 

 thoracic appendages (figs. 47, 48, md. n. 2 — 6). This number 

 corresponds approximately to the number of supra-branchial 

 sense organs in Vertebrates, and affords us a satisfactory ex- 

 planation of their relation to the gill-arches and their absence 

 from the trunk in the early stages of development. 



I have already pointed out in my paper on the " Origin of 

 Vertebrates" how the cephalo-thoracicneuromeres of Scorpions 

 and Limulus are comparable with the entire brain of Ver- 

 tebrates. This is clearly shown by the similarity in the grouping 

 of the neuromeres, and by the modifications these groups 

 have undergone. 



The modifications and homologies of the three segments 

 constituting the fore-brain of Limulus and Vertebrates have 

 already been considered. The mid-brain of Vertebrates is 

 generally conceded to consist of a single neuromere, distinct 

 from those in front and behind, and characterised by the fact 

 that it is provided with the only pair of cranial nerves arising 

 from the neural surface of the brain. We find the same isolated 

 neuromere in Arthropods, namely, the one belonging to the 

 antennal segment of Insects and Myriapods, and the cheliceral 

 segment of Arachnids. In the Arachnids it is easily dis- 

 tinguished from the true fore-brain on the one side and from 

 the post-oral neuromeres on the other. Owing to the position 

 of the chelicerae, close together in front of the mouth, their 

 nerves invariably arise from the neural surface of the neuro- 

 mere, while all the others, except, perhaps, in the vagus 

 region, arise from the sides. 



