Il8 THE ORIGIN OF VERTEBRATES 



In two respects the retina of the lateral eyes of vertebrates differs from that 

 of all arthropods, for it possesses a special supporting - structure, the Mullerian 

 fibres, which do not exist in the latter, and it is developed in connection with 

 a tube, the optic diverticulum, which is connected on each side with the main 

 tube of the central nervous system. These two differences are in reality one 

 and the same, for the Miillerian fibres are the altered lining cells of the optic 

 diverticulum, and this tube has the same significance as the rest of the tube of 

 the nervous system ; it is something which has nothing to do with the nervous 

 portion of the retina but has become closely amalgamated with it. The explana- 

 tion is. word for word, the same as for the tubular nervous system, and shows that 

 the ancestor of the vertebrate possessed two anterior diverticula of its alimentary 

 canal which were in close relationship to the optic ganglion and nerve of the 

 lateral eye on each side. It is again a striking coincidence to find that 

 Ai-temia, which with Branchipus represents a group of living crustaceans most 

 nearly allied to the trilobites, does possess two anterior diverticula of the gut 

 which are in extraordinarily close relationship with the optic ganglia of the 

 retina of the lateral eyes on each side. 



The evidence of the optic apparatus of the vertebrate points most remarkably 

 to the derivation of the Vertebrata from the Palfeostraca. 



