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PERIPHERAL NERVES AND GANGLIA. 



In the arachnids (scorpion), four abdominal neuromeres have migrated for- 

 ward to unite with the hindbrain. Of these four, the last one is a true branchial 

 neuromere. 



In vertebrates, all the branchial neuromeres have fused with the hind- 

 brain, probably in some such manner as that indicated in Fig. 68. The 

 hypobranchial nerves united to form the hypoglossus, having the peculiar distri- 

 bution indicated above, although in a more exaggerated form. The neural roots 



FIG. 82. Muscles and distribution of nerves in the first gill of Limulus. The appendage is flexed upon the 

 abdomen, and is seen from the neural side. a. e.9, Abdominal endochondrite; fc.c.8 and be. 9, branchial cartilages 

 of operculum and first gill; i.l., inner lobe of gill; rn.l. median lobe of gill o.l., outer lobe of gill. 



MUSCLES: a.b.m.9, Abductor muscle of gill; b.t.m., branchio-thoracic muscles; e.b.m.9, external branchial 

 muscle; i.b.m.9, internal branchial muscle; i.l.m., inner lobe muscles; l.a.m., longitudinal abdominal muscles; o.l.m., 

 outer lobe muscles. 



NERVES: a.g., First abdominal ganglion; e.b.n., external branch of neural nerve; g.n., branch of neural nerve 

 supplying gill book; h.n.9, haemal nerves; i.b.n., internal branch of neural nerve; i.n.9, intestinal nerve (two branches 

 are shown, a posterior and an anterior one); in.n.,9 integumentary branch of haemal nerve; l.s.n., hypo-bran- 

 chial nerve; m.b.n., median branch of neural nerve; m.n. 9, neural nerve; s.c.n.,9 segmental cardiac nerve; 

 v.c., ventral cord. 



united with one another, and with the posterior neural roots of the vagus, as they 

 have to a certain extent in the scorpion, to form the series of nerves supplying the 

 gill arches. The nerves supplying the important sense organs in the group of 

 modified vagal appendages, gave rise to the lateral line nerve; and the combined 

 cardiac and intestinal components, that in the arachnids are confined to this 

 region, gave rise to the corresponding elements in the vertebrates. 



We need not carry this comparison any farther, for the conditions are ex- 

 tremely complicated, and there are many variations peculiar to each class. But 

 that this entire region has undergone the same kind of changes in vertebrates that 



