THE EVIDENCE OF THE ORGANS OF VISION 109 
Of these two parts we have already seen that the second is to 
all intents and purposes a compound retina of a crustacean eye, and 
seeing that the single-layered epithelial tube is continuous with the 
single-layered epithelial tube of the central nervous system—1.¢, with 
the cephalic part of the gut of the arthropod ancestor—it follows with 
certainty that the ancestor of the vertebrates must have possessed 
two anterior diverticula of the gut, with the wall of which, near the 
anterior extremity, the compound retina has amalgamated on either 
side, just as the infra-cesophageal ganglia have amalgamated with 
the ventral wall of the main gut-tube. In this way, and in this way 
alone, does the interpretation of the structure of the vertebrate lateral 
eye harmonize in the most perfect manner with the rest of the con- 
clusions already arrived at. 
The question therefore arises :—Have we any grounds for believing 
that the ancient forms of primitive crustaceans and primitive arachnids, 
which were so abundant in the time when the Cephalaspids appeared, 
possessed two anterior diverticula of the stomach, such as the con- 
sideration of the vertebrate eye strongly indicates must have been 
the case ? 
The beautiful pictures of Blanchard, and his description, show 
how, on the arachnid side, paired diverticula of the stomach are 
nearly universal in the group. Thus, although they are not present 
in the scorpions, still, in the Thelyphonide, Phrynide, Solpugide, 
Mygalide, the most marked characteristic of the stomach-region is 
the presence of four pairs of ccecal diverticula, which spread laterally 
over the prosomatic region. In the spiders the number of such 
diverticula increases, and the whole prosomatic region becomes filled 
up with these tubes. Blanchard considers that they form nutrient 
tubes for the direct nutrition of the organs in the prosoma, especially 
the important brain-region of the central nervous system. He points 
out that these animals are blood-suckers, and that, therefore, their 
food is already in a suitable form for purposes of nutrition when it 
is taken in by them, so that, as it were, the anterior part of the gut 
is transformed into a series of vessels or diverticula conveying blood 
directly to the important organs in the prosoma, by means of which 
they obtain nourishment in addition to their own blood-supply. 
The universality of such diverticula among the arachnids makes 
it highly probable that their progenitors did possess an alimentary 
canal with one or more pairs of anterior diverticula, In the 
