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. 
