24 Dr Gaskell, The Origin of Vertebrates. [Nov. 25, 



derived from the branchial bars of Limulus, so also does the con- 

 sideration of the muscular system lead to the same conclusion, 

 viz. that the branchial muscles are derived from the muscles of 

 the branchial appendages of Limulus, and therefore to the further 

 conclusion that the Branchiomerie is due to the segmentation of 

 appendages: in other words, that V m , VII, IX and X were all 

 originally appendage nerves ; the proof of this will be given 

 later. 



Again, we find that histological considerations point in a 

 most striking way to the same conclusion : for we find in Verte- 

 brates that these splanchnic muscles are not of the same struc- 

 ture as the somatic, and this difference of structure is most 

 striking, as shown by Schneider, when we reach the lowest 

 Vertebrate — the Ammocoetes. Here indeed we see the evidence 

 of its ancestry in that its branchial muscles are to a large extent 

 composed of tubular fibres such as are found in Invertebrates, 

 very similar to many found in scorpions, and not found in Verte- 

 brates. These muscles are as significant a finger-post as the 

 pineal eye, and show their phylogenetic meaning by disappearing 

 entirely at transformation. 



The history of the respiratory system and of the IXth 

 and Xth nerves. 



Perhaps the best known phylogenetic development of any 

 system in the Vertebrates is that of the respiratory system, for 

 we can trace the lungs back to the swim bladder and pass from 

 the air breathing to the gill breathing Vertebrate, and from thence 

 to the Elasmobranchs with indications of extra-branchial carti- 

 laginous bars in addition to the internal cartilaginous bars, until 

 we reach the oldest known vertebrate respiratory system, that of 

 Ammoccetes with its extra-branchial cartilaginous basket-work, 

 and its segmental branchiae dependent from the basket-work into 

 the large open respiratory chamber. Throughout the water 

 breathing Vertebrates the innervation is the same, the VHth nerve 

 supplying the 1st or hyo-branchial segment, the IXth the second 

 or 1st branchial segment, the Xth the remaining segments and 

 throughout, even in the highest lung-breathing animals, the 

 respiratory centre still keeps its primitive position in connection 

 with the origin of the IXth and Xth nerves. 



In Ammocoetes we find in strict accordance with the arrange- 

 ment and nature of the branchial cartilages and muscles that the 

 separate branchiae may be considered as paired branchial ap- 

 pendages hanging suspended into a chamber so as to leave a 

 narrow slit between opposite pairs for the passage of water and 

 food. Further, we find that the blood circulates in these branchiae 



