CHAPTER VI. 



PERIPHERAL NERVES AND GANGLIA. 



I. COMPONENTS OF A NEUROMERE. 



In my first paper "On the Origin of Vertebrates," 1889, it was maintained 

 that the primitive arthropod neuromere was a complex structure, consisting of 

 two segments, four pairs of nerves, and a segmented middle cord. 



While I have seen no reason to change my view as to the composite nature 

 of primitive neuromeres, I do not now regard the ancestral arthropod as an elon- 

 gated worm-like animal of many like metameres, but as a small-bodied one of 

 about three imperfect segments. In the arachnid and crustacean descendants 

 of this stock, the evolution of neuromeres, as we have explained elsewhere 

 (Chap. XIII) was a gradual process that advanced with the successive additions 

 of new groups of unlike metameres. 



II. NERVES OF THE DIENCEPHALON AND MESENCEPHALON. * 

 A. Neural Nerves. 



In Limulus, there are six pairs of thoracic neural nerves. (Figs. 36-39.) 

 The third nerve is typical. (Fig. 79.) It divides, soon after leaving the brain, 

 into three sets of nerves. The gustatory nerves, three in number, are ganglionated 

 and terminate in the numerous sensory buds of the mandibular spines. They are 

 absent, or very minute, in the sixth pair of legs. The anterior and posterior 

 entocoxal nerves, a.e.n., and p.e.n. are motor and supply the tergocoxal muscles; 

 the median entocoxal nerve, m.e.n., supplies the sensory knobs of the coxopodite. 

 The main pedal nerve, consisting of two principal branches, supplies the muscles 

 and sense organs of the appendage. 



The Flabellum. There are a few minor differences between the six pairs 

 of pedal nerves; the most important is an enormous enlargement of the 

 median entocoxal nerve of the sixth leg to form the nerve of the flabellum. (Fig. 

 80, flab.) 



The flabellum is a large spatulate organ attached to the outer side of the 

 coxal joint of the sixth leg. It is first seen in the embryos as a rounded knob, 

 lateral to the sixth leg, and quite separate from it. Hence it has the same rela- 

 tion to the outer side of the appendage that the mandibular placode has to the 

 inner. (Figs. 141-148.) There are indications of flabellar placodes on the other 



1 For nerves of the prosencephalon see Chapters VIII to X. 



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