THE EVIDENCE OF THE ORGANS OF VIS I OX 1 09 



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 — i.e. 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 Thelyphonidae, Phrynidas, Solpugidae, 

 Mygalidse, 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 rilled 

 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 



