MOEPHOLOOY OF BRAIN AND SENSE ORGANS OP LIMULUS. 71 



carefully, I am not aware that their existence has been 

 described in fishes, but it is hoped they will soon receive the 

 careful attention they deserve. However, if we can regard the 

 course and termination of these peduncles in mammals as 

 typical for the Vertebrates, it is obvious that they correspond 

 to the divergent roots of the parietal eye nerve of Limulus. 

 On turning to fig. 43, it is evident that the parietal eye could 

 be moved back to the position it occupies in mammals under 

 the backwardly projecting lobes of the hemispheres, for the 

 diverging roots (/. n. m. e.) could be slipped over the hemi- 

 spheres, and when shortened would form nerve-bands or 

 peduncles encircling the"tween brain "and terminating in the 

 infundibulum. By turning to figs. 24, 25, and 59, it is seen 

 that the olfactory organ would not stand in the way of this 

 change provided it were made early enough, since the olfactory 

 organs do not unite in the median line, and the median olfactory 

 nerve is not formed, till long after the parietal eye has assumed 

 its final position. The diff'erence, then, between the position 

 of the parietal eye in Limulus and Vertebrates is more apparent 

 than real, and is probably due to the fact that Limulus moves 

 about on its neural surface, and the parietal eye, to be of any 

 use, must be on the opposite side ; the extent to which it has 

 wandered from its original position on the cephalic lobes is 

 shown approximately by the great length of its nerve in the 

 adult. In Vertebrates the neural surface is turned upward, 

 hence the parietal eye can remain nearer its original position 

 than in Limulus. 



The, at first sight, apparently inexplicable position of the 

 parietal eye in Limulus, and its connection with the infundi- 

 bulum, are seen, therefore, to be in harmony with Vertebrate 

 anatomy as soon as we recognise that the true roots of the 

 nerve to the parietal eye in Vertebrates do not terminate in 

 the roof of the " tween brain," but in the infundibulum. 



The Third Ventricle. — It is evident that in Limulus the 

 space between the lateral ventricles and the cavity of the in- 

 fundibulum corresponds to the third ventricle of Verte- 

 brates (compare figs. 25 and 43). The resemblance appears 



