43 2 



THE POPULAR SCIENCE MONTHLY 



arachnids is identical with that of the veitebrates, and lies on the same 

 side of the body. It affords us the fixed base line of comparison. 

 (2) That in embryo vertebrates there are at least three pairs of primi- 

 tive jaws that arise from the neural surface ; they are gradually shifted 

 from their old position to a new one on the opposite side of the head. 



Fig. 7. Semi-diagrammatic Median Sections of an Arachnid and a Verte- 

 brate, showing the location of the principal organs and their probable relations ; 

 cerM., cerebellum ; cer. h., cerebral hemispheres ; g. h., ganglion of parietal, or pineal, 

 eye ; hy., hypophysis ; inf., infundibulum ; ol. 1., olfactory lobes ; p. eye., parietal or 

 pineal eye ; s. vas., saccus vasculosus ; thyr., thyroid. 



(3) That the old mouth and oesophagus of the invertebrates is still 

 present, a useless, heretofore unintelligible, rudiment, in its proper 

 place in the floor of the brain of all vertebrates. This old passageway 



