48 NERVOUS SYSTEM AND SENSE-ORGANS 



only conclude that this half can see clearly in water, while the 

 upper half has been so modified that distinct vision in air has 

 also become possible. 



Some of the Reptiles possess a more or less degenerate third 

 or pineal eye on the top of the head (fig. 1061). It is connected 

 with the roof of the 'twixt-brain. There seems good reason to 

 believe that the ancestral Vertebrates had at least one visual 

 organ in this position, probably serving as a means of detecting 

 enemies attacking from above, a contingency to which aquatic 

 forms are peculiarly liable. We may perhaps compare it with the 

 internal brain-eye of the Ascidian tadpole, which also is unpaired 

 and dorsal. 



