Evolution of Animals 441 



in the upper brain lobes as the important cognito-cogitic cen- 

 ters, and four in the lower brain lobes as the important biotic 

 centers. So the correlating cognito-cogitic center or brain, 

 in the main line of invertebrate-vertebrate ascent, represents 

 the result neither of a segmentation nor cephalization process, 

 but of a sensorization process, or is a center for increasingly 

 rapid condensation and correlation of all external sensory 

 impressions. 



We would pass beyond the scope of the present work were 

 we to deal with the circular nerves of nemerteans that recall 

 interesting homologies with vertebrates. But we would merely 

 add that the abundant and serially arranged pairs of nerve 

 threads that connect the lateral nerve cords with each other 

 and with the ventral pairs (Fig. 17a) cannot but call up a close 

 parallel with the spinal nerves and nerves of the lateral line, 

 as well as the spinal-sympathetic nerves, in vertebrates. 



In proceeding to a study of the nervous system in Myxine, 

 the writer should state that, while the existing cyclostomes 

 are regarded by him as nearly direct types in the line of verte- 

 brate evolution, forms must have existed that we might view 

 as leading up from the nemerteans to an ancestor that was 

 common to the living cyclostomes which are now lateral and 

 somewhat degraded offshoots, and to a direct series that led 

 upward through paracyclostomatous types toward the Apoda 

 or Cseciliada of the Batrachia. But the Cyclostomata repre- 

 sent living material that alone is to hand. 



Reference has already been made to the median nasal sac 

 on which much work needs to be expended. 



The eye of cyclostomes is not greatly advanced over that 

 of the most complex nemerteans. But the evolutionary con- 

 nection between the pineal and parapineal eyes, alike with 

 nemerteans on the one hand and higher vertebrates on the 

 other, deserves notice. As Blirger points out for the former, 

 the eyes may not only at times be somewhat sunk below the 

 skin, they may tend to show an uneven position or number, 

 or a clustering into two or more. Now in fresh- water or land 

 genera like Geonemertes and Tetrasternma, that usually have 

 four eyes, we have an exact morphological basis for origin 



