556 



NA TURE 



[OcTOIiF.R 8, 1896 



what is the most marked characteristic of the vertel)rate cranial 

 nerves, viz. that they are divisible into two sets corresponding 

 to a double segmentation in the head region. The one set, 

 consisting of the \'th, Vllth, I.Xth, and Xth nerves, supply the 

 muscles of the branchial or visceral segments; the other set, 

 consisting of the Ilird, IVth, \'Ith, and Xllth nerves, the 

 muscles of the somatic segments. Further, we see that the 



Branchial 

 Cartilages 



of Appendages and Cartilages. 



nerves supplying the branchial segments, like the nerves supply- 

 ing the appendages in Limulus, are mixed motor and sensory, 

 while the nerves supplying the somatic segments are all purely 

 motor, the corresponding sensory nerves running separately as 

 the ascending root of the fifth nerve ; so also in Limulus, the 

 nerves supplying the powerful body muscles arise separately 



from those supplying the appendages, and also are quite 

 separate from the purely sen.sory or epimeral (Milne Edwards, 

 " Recherches sur I'Anatomie des Limulus," Ann. des Sk. Nat., 

 Sth ser. ) nerves which supply the surfaces of the carapace in the 

 prosomatic and mesosomatic regions. Finally, the researches 

 of Hardy (/Vi//. Trans. J\ov. Sa\, 1894) have shown that the 



NO. 1406, VOL. 54] 



motor portion of these appendage nerves, just like the nerves ot 

 the branchial segmentation in vertebrates, t'.i: the motor part of 

 the trigeminal, of the facial, of the glosso- pharyngeal, and of the 

 vagus, arise from nerve centres or nuclei quite separate from 

 tho.se which give origin to the motor nerves of the somatic 

 muscles. The phylogeiietic history, then, of the cranial nerves 

 points directly to the conclusion that the Vth, VHth, IXth, and Xth 

 nerves originally innervated structures of the nature of arthropod 

 appendages. 



We can, however, go further than this, for we find, as we 

 trace downwards throughout the vertebrate kingdom the 

 structures supplied by the.se nerves, that they are divisible into 

 two well-marked groups, especially well seen in Ammoccetes, 

 viz. : — 



(1) A posterior group, viz. the Vllth, LXth, and Xth nerves, 

 which arise from the medulla oblongata and supply all the 

 structures within a branchial chamber. 



(2) An .anterior group, viz, the Vth nerves, which arise from 

 the hind brain and supply all the structures within an oral 

 chamber. 



Sulichord,il Carti- 

 laginous Ligaments 



. 6. — .\niniocoetes. Nerves of visceral segme 

 ree Figures vj-v.^ ^ Prosomatic appendage 

 )sonratic appendage or opercular appendage n 

 lining mesosomatic appendages and nerves ; : 

 istoma in Eurypterus. 



Its and cartilages. 



id nerves ; ix, Xi . . .= 

 i=ChiIaria in Limulus. 



The reason for this grouping is seen when we turn to Limulus 

 and its allies, for we find that the body is always divided into a 

 prosoma and mesosonia, and that the appendage nerves are 

 divisible into two corresponding well-marked groups, viz. : — 



(1) A posterior or mesosomatic group, which arise from the 

 mesosomatic ganglia and supply the operculum and branchial 

 appendages. 



(2) Au anterior or prosomatic group, which arise from the 

 prosomatic ganglia and supply the oral or locomotor appendages. 



Comparison of the Branchial Appendages of Limulus, 



Eurypterus, d-<., with the Branchial Appendages of 



Ainniocates. Meaning of the IXth and Xth Ncn-es. 



We will first consider the posterior group— the Vllth, IXth, 



and Xth nerves— and of these I will take the IXth and .Xth 



nerves together, and discuss the Vllth separately. These 



nerves are always described as supplying in the fishes the 



muscles and other tissues in the walls o( a .series of gill-pouches, 



so that the respiratory chamber is con.sidered to consist of a 



series of pouches, which open on the one hand into the 



alimentary canal, and on the other to the exterior. .Such a 



