MOEPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 5 



become so extensive as to enfold not only the optic ganglia, 

 but the eyes and the fore-brain as well. A cerebral vesicle is 

 thus formed, from the floor of which arise the fore-brain and 

 the optic ganglia, and from the roof a tubular outgrowth, at 

 the end of which lie the inverted retinas of the parietal eye. 

 Such a condition is found only in Arachnids and Vertebrates; 

 and in my judgment this fact, when all its bearings are con- 

 sidered, aflbrds as trustworthy evidence of relationship as the 

 presence of a uotochord or of gill-slits. 



It was further shown (1) that the lateral eyes of Limulus 

 could be compared with the lateral eyes of Vertebrates; (2) 

 that there is in Arachnids a cartilaginous endocranium similar 

 in position, shape, and development to the primordial cranium 

 of Vertebrates ; (3) that there is in scorpions and in other 

 Arthropods a subneural rod similar to the notochord of Ver- 

 tebrates; (4) that in the Arachnids and in the Vertebrates 

 the brain contains approximately the same number of neuro- 

 meres; it is divided into the same number of regions, i.e. 

 fore-brain, mid-brain, hind brain, and accessory brain ; and 

 while in each region there is a diflferent number of neuro- 

 meres, i. e. 3, 1, 5, 2 to 4, the number in the corresponding 

 regions in Vertebrates and Arachnids is very nearly the same; 

 (5) the nerves of each brain region in both Vertebrates and 

 Arachnids show in a general way the same relation to sense 

 organs, the same ganglia, and the same distribution and fusion 

 with one another; (6) finally, there is a striking similarity 

 between the cephalo-thoracic shields of Arthropods and the 

 cephalic bucklers of the earliest fishes, such as Zenaspis, Both- 

 riolepis, Pteraspis, Auchenaspis, &c. The shape and micro- 

 scopic structure of the shields, and the arrangement of the 

 eyes upon them, are practically the same in both groups. 

 The three median and two lateral eyes of Cephalaspis 

 Campbelltonensis as figured by Whiteaves (^ Trans. Roy. 

 Soc. Canada,' vol. vi, 1888, pi. x) have exactly the same 

 arrangement as those of Limulus. I have carefully revised 

 the palseontological aspect of the subject, and I hope in a 

 separate paper to give it the careful consideration it deserves. 



