SUMMARY AND CONCLUSION. 173 



The primary sensory functions of the marine arachnids were, therefore, 

 visual and gustatory, and the main centers for these functions lie respectively in 

 the optic ganglia and the primitive cerebral hemispheres. 



In the higher marine arachnids, and toward the beginning of the ostracoderm, 

 or primitive vertebrate stage in phylogeny, the olfactory function, as a secondary 

 aid to nutritive and sexual activities, became definitely localized, and the most 

 anterior section of the forebrain was successfully preempted as the main olfactory 

 center. 



The auditory function was definitely localized at a much later period than 

 any of the three preceding ones, and it has probably for that reason never suc- 

 ceeded in creating for itself a definite, sharply circumscribed, brain region. 



