110 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 
Fic. 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 
