MOEPHOLOGY OF BRAIN AND SENSE OEGANS OF LIMULUS. 77 



distal end splits up into four branches, two of which plunge 

 directly into the median diverticulum or endoparietal eye (fig. 

 63, n. en. p. e.), and the other two pass to the paired retinas of 

 the ectoparietal eyes (figs. 63 — 65, ec. p. e.). Two delicate 

 nerve-strands split off at a very early period from the proximal 

 end of the primitive eye-tube and its diverging arms (figs. 35, 

 /. n. m. e. and fig. 36, r. m. e. n.). Compare also surface view 

 (figs. 46 and 47, r. m. e. n.). These two nerve-roots subse- 

 quently divide (fig. 49), so that there are at least four main 

 roots to the nerve, not including the epiphysis. Two of these 

 roots terminate in conspicuous medullary nuclei attached to the 

 horseshoe-shaped medullary core in the interior of the semi- 

 circular lobes {p. e. c). From each nucleus a delicate strand 

 passes laterally to the peduncle of the lateral optic ganglia 

 {s.t.p.). 



The epiphysis, or what is left of the median eye-tube after 

 splitting off the roots to the parietal eye-nerve, remains for 

 some time unchanged, as in fig. 43, m. e, t., also figs. 46 — 49 

 and 68 — 70. Finally its lumen disappears, and several 

 swellings appear in it, composed of small nuclei like those 

 in the cerebral hemispheres. In specimens four or 

 five inches long these swellings may be seen em- 

 beddedinthe brain evelope, and extending downward 

 from the root of the median olfactory nerve toward 

 the parietal eye-nerve (fig. 49, m. e. ^). After this stage 

 the epiphysis loses altogether its connection with the root of 

 the parietal eye-nerve, and then disappears completely. 



We have in the parietal eye seven distinct struc- 

 tures, which should be constantly borne in mind in 

 making any comparison with the parietal eye of 

 Vertebrates. (1) The primary brain diverticulum or 

 primitive parietal eye-tube; (2) the paired, and (3) the 

 unpaired diverticula at its distal end; (4) the solid 

 nerve-stalk to these diverticula; (5) the epiphysis, or 

 remnant of the primitive parietal eye-tube after 

 splitting off the (6) roots of the median eye-nerve, and 

 finally (7) the large blood-vessel accompanying the 



