258 DR WALTER H. GASKELL. 



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 harmonise in the most perfect manner with the rest 

 of the conclusions given in this series of papers. 



The question therefore arises — Have we any grounds for be- 

 lieving 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 consideration 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 Thelyphonidfe, Phrynidse, 

 Solpugidse, Mygalidse, tbe most marked characteristic of the 

 stomach region is the presence of four pairs of ctecal diverticula, 

 which spread laterally over the prosomatic region. In the 

 spiders the number of such diverticula increase, 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 con- 

 veying 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 the progenitors of the Arachnids 

 did possess an alimentary canal with one or more pairs of 

 anterior diverticula. In the vertebrate, however, the paired 

 diverticula are associated with a compound retina, a combination 

 which does not occur among living arachnids ; we must, there- 

 fore, examine the crustacean group for the desired combination, 

 and naturally the most likely group to examine are the 

 Phyllopods, especially such primitive forms as Branchipus and 

 1 Blanchard, L' organization clu Eegnc animal — Arachnides. 



