ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 343 



refractive rod-like thickenings, which resemble those in the 

 superficial median eyes. 



The extraordinary condition described above can be explained 



Fig. 10. — Fore- and mid-brain of young larva of Limulus (just hatched), 

 seen from neural surface ; constructed from sections and dissections. — 

 c. 11^. Posterior commissure of brain, e. Eye to third brain-seg- 

 ment, y-^. Optic ganglia, g, ti*. Spinal ganglion on fourth neural 

 nerve, g. ce. n. Ganglion to stomodeal nerves, h, u"-'^. Haemal nerves 

 to second, third, and fourth neuromeres. Z'-^. Three ganglionic lobes 

 to ganglion of lateral eyes. iV'-^. First four neuromeres. n. e. Nerve 

 to rudimentary eye of third segment, n. I. e. Nerve to lateral eyes. 

 n. 7i^. Neural nerve to fourth neuromere. up. and g, v^. Neuropore. 

 7i. ce. Stomodeal nerves, oe. Oesophagus, .r. Small nerve, extending 

 backwards from posterior brain commissure. 



by supposing that two distinct pairs of segmental eyes are fused 

 in the primitive eye-bulb ; one pair belonging to the first, the 

 other to the second brain-segment. Compare the diagrammatic 

 figs. A and B, Fig. 7. In confirmation of this view it may be 

 added — (1) at an early stage there are traces of two pairs of 



