ON THE OKIGIN OF VERTEBRATES FROM ARACHNIDS. 341 



be derived from the median eye of Scorpio and Limulus. As 

 we shall now show, the structure of the median eye of Limulus 

 supports this conclusion In Limulus the optic tubes of the 

 median eye unite in such a way as to form a x-tube, with the 

 median pore at the junction of the arms and upright (compare 

 Figs. 7 and 10). This condition is easily understood if we 

 imagine that as each tube was bent toward the median line its 

 mouth was gradually carried toward the distal end of the tube. 

 The median pore represents not only the last trace of the coa- 

 lesced ganglionic invaginations, but also the last point 

 where the enclosed brain is attached to the surface 

 ectoderm or communicates with the exterior. Nos. 

 1 — 4, Fig. 7, represent, in a diagrammatic way, the forma- 

 tion of the optic tube of the median eyes ; at first they are 

 apparently not connected with the brain at all, but later the 

 invagination involves both the brain and optic ganglion. It is 

 important to notice what a great variety of conditions might 

 arise by slight variations of this process. The base of the 

 X-shaped optic tube soon splits, in a way not thoroughly 

 understood, into three nerves : (1) a delicate anterior pair 

 (Fig. 7, B, and Fig. 10, n'), which arise from the very anterior 

 lobe of the brain and unite with the eye-stalk some distance 

 in front of the neuropore ; after joining the stalk they may be 

 followed along its sides to the eyes (Fig. 8, C, n',) ; (2) a 

 posterior impaired nerve (Fig. 10, w'^.), which for a long 

 time is tubular at its base. Here it divides into two smaller 

 arms (^^.), which join the sides of the second brain-segment. 

 Towards the eyes the tube is gradually converted into a small 

 bundle of pigmented nerve-fibres (Fig. 8, n^.). The structure 

 of the median eye and its stalk is well shown by the series 

 of cross sections in Fig. 8. 



The eye-stalk extends through the anterior part of the body 

 to the dorsal surface, and then expands into a bulb-like thick- 

 ening. The outer wall of the bulb develops two groups of 

 cells filled with black pigment; they give rise to the median 

 eyes proper, and are supplied by the median pigmented nerve, 

 n~. The inner wall of the bulb is filled with white pigment 



