70 WILLIAM PATTEN. 



wards on to the haemal surface^ and form cavities communi- 

 cating directly with the third and the lateral ventricles ; 

 (3) their relations to the median or parietal eye are apparently 

 the same in both Limnlus and Vertebrates. 



Concerning the last faot^ it seemed at first impossible to 

 satisfactorily account for what is apparently a great difference 

 in the position and attachment to the brain of the parietal 

 eye in Scorpions, Limulus, and Vertebrates. But these differ- 

 ences are very easily explained, and when thoroughlyunderstood, 

 help to strengthen the comparisons already made. For instance, 

 in Vertebrates the parietal eye arises from the middle of the 

 roof to the thalamencephalon,back of the cerebral hemispheres. 

 In Limulus it seems to be about as far as possible from its 

 apparent position in Vertebrates, for it lies in front of the 

 cerebral hemispheres, and has its roots inserted into what, in 

 Vertebrates, would be the floor of the infundibulum 

 (figs. 43 — 49). But on looking at the spider's brain (fig. 61), 

 which for the point under consideration will serve as well as 

 that of the scorpion, we see that the parietal eye lies 

 back of what corresponds to the cerebral hemispheres in 

 Limulus {c. h.); and this position, as shown by the whole course 

 of development, seems to be the primitive one for Arachnids. 

 But by elongating its divergent nerve-roots the 

 median eye can be moved forward in front of the 

 cerebral hemispheres, so that its roots, instead of 

 describing a half- circle around the posterior neural 

 surface of the fore-brain, lie like an inverted Y on 

 its anterior haemal surface; the points of attach- 

 ment, however, will be the same in either case. In 

 Limulus this very change has taken place, owing to the move- 

 ment of the eye farther and farther forward on to the haemal 

 surface. It is a very significant fact that in mammals there 

 are two bands, the peduncles of the parietal eye, that extend 

 around the ^'tween brain" and terminate in the posterior 

 wall of the infundibulum, in the " corpora albicans." I 

 have seen something similar to these bands in the brain of 

 Petrorayzon, but have not yet had time to work them out 



