I IO 



THE ORIGIN OF VERTEBRATES 



vertebrate, however, the paired diverticula are associated with a 

 compound retina, a combination which does not occur among living 

 arachnids ; we must, therefore, examine the crustacean group for the 

 desired combination, and naturally the most likely group to examine 

 is the Phyllopoda, especially such primitive forms as Branchipus and 

 Artemia, for it is universally acknowledged that these forms are 



Al 



.— rt.gl 



Fig. 45. — Section through one of the two Anterior Diverticula of the Gut 

 in Artemia and the Retinal Ganglion. 



The section is through the extreme anterior end of the diverticulum, thus cutting 

 through many of the columnar cells at right angles to their axis. AL, gut 

 diverticulum ; rt. gl., retinal ganglion. 



the nearest living representatives of the trilobites. If, therefore, it 

 be found that the retina and optic nerve in Artemia is in specially 

 close connection with an anterior diverticulum of the gut on each 

 side, then it is almost certain that such a combination existed also 

 in the trilobites. 



My friend Mr. W. B. Hardy has especially investigated the 

 nervous system of Artemia. In the course of his work he cut serial 



