118 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 Milerian 
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 
Artemia, 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 Paleostraca. 
