236 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



neither hypothesis is at all in harmony with the facts. 

 Both explanations ignore a third much simpler one, that 

 the anterior extremity of the primitive form must have 

 infallibly differed sufficiently from the middle region to 

 explain any difference between cranial and spinal nerves. 

 And such a difference would, of course, be considerably 

 emphasised by the production of such distinctively verte- 

 brate characters as gill slits and lateral sense organs, the 

 growth of both of which would, as has been previously 

 pointed out, involve marked changes in the brain and 

 anterior or cranial nerves. In a sense, the view that the 

 head is younger than the body is certainly true, if we 

 overlook the fact that it represents the modified anterior 

 extremity of the primitive form. For these modifications 

 have been so great that they largely constitute the differ- 

 ence between vertebrates and invertebrates. The meta- 

 merism of the body is a well-marked invertebrate 

 character that is lacking in the head. Further, such 

 structures as gill arches and lateral sense organs, being 

 distinctively vertebrate characters, are necessarily "new." 

 This has been well shown by Ayers to be the case with the 

 vertebrate auditory organ, which we now know has been 

 evolved within the vertebrate series, is therefore a charac- 

 teristically vertebrate structure, and must have taken a 

 considerable part in the production of the head. In this 

 connection Amphioxus is very interesting. It possesses 

 a well marked and modified anterior extremity, but the 

 condition of its notochord, somatic muscles, and nervous 

 system, distinctly shows that it has no head properly 

 so-called, and it seems to me that some such condition as 

 this probably characterised the anterior extremity of the 

 primitive vertebrate. We may therefore conclude that 

 the cranial nerves, as such, are phylogenetically younger 

 than the spinal nerves ; or, to put it another way, the 



