30 Dr GasJcell, The Origin of Vertebrates. [Nov. 25, 



panied into the capsule by a diverticulum of the liver and 

 generative organ. 



In the Limulus the liver and generative organ, which surround 

 the central nervous system from one end of the body to the other, 

 do not penetrate into any of the appendages, whether thoracic or 

 abdominal, with the single exception of the flabellum. 



In the Ammocoetes the peculiar glandular and pigmented tissue 

 which we have already recognised as the remains of the liver and 

 generative organ, which surrounds the brain and spinal cord, does 

 not penetrate into the velar or other appendages with the single 

 exception of the auditory capsule where it enters with and sur- 

 rounds the auditory nerve just as in Limulus. 



Although this organ is not apparent in Eurypterus, yet we can 

 see how the Vlllth nerve came into closer relations with the 

 Vllth or opercular nerve than with the Vth as in Limulus, when 

 the scorpions arose from a Limulus-like form ; for in the scorpion 

 we see that a large organ similar in structure to the lyriform 

 organs, evidently a special sense organ with a nerve of its own, 

 viz. the pecten, is situated in closer relationship to the operculum 

 than to the thoracic appendages ; so that looking at it superficially 

 Ray Lankester considers it as the second abdominal appendage. 

 In reality its nerve leaves the nervous system anteriorly to the 

 nerve of the operculum, and the pecten therefore belongs to a 

 segment immediately anterior to the operculum, and is in all 

 probability simply the flabellum of Limulus enlarged and pushed 

 downwards to gain room, its proper metameric position being 

 shown by the origin of its nerve from the central nervous system 

 between the opercular and last locomotor appendage nerves. 



In Limulus the auditory apparatus is enclosed in a chitinous 

 envelope, not yet become cartilaginous, and as already mentioned 

 the cartilaginous auditory capsule and the trabeculae of Ammocoetes 

 are more recent formations than the branchial cartilages, which 

 latter are already found in Limulus. 



The history of the cranial segments and nerves. 



From what has been said it follows that the mystery of the 

 vertebrate cranium and cranial nerves is now solved, for we see 

 that the question of the bony segments can be traced back to 

 Petromyzon and so to Ammocoetes, with its direct genetic relation- 

 ship to Eurypterus, and that the cranial nerves are in their origin 

 the nerves of the prosomatic and mesosomatic appendages of 

 Eurypterus, and so of Limulus. 



We may tabulate them as follows : 



