344 WILLIAM PATTEN. 



ganglionic pits, one of which was small and disappeared 

 quickly, so that its history could not be determined. (2) 

 Although there are only three distinct nerves in the eye-stalk, 

 they rejiresent four completely fused nerves, for the colourless 

 nerves are still clearly enough paired, while the median one 

 was undoubtedly paired originally^ as shown by the diverging 

 arms terminating in paired ganglia. (3) The anterior pair of 

 nerves arise from a distinct crescent-shaped brain-lobe (with 

 small dark nuclei), which is undoubtedly homologous with a 

 similar lobe in Scorpio, and which there represents the first 

 brain- segment ; the posterior pair arise from another lobe, 

 which probably represents the second brain-segment. Thus 

 the diverticulum of the median eye-bulb represents, in all 

 probability, a pair of eyes belonging to the first brain- segment. 



We will merely note in passing that in some Trilobites on 

 the anterior portion of the glabellum are three eye-like spots, 

 which may possibly represent the three fused ocelli of Limulus. 



In comparing the median eye of Limulus with the pineal 

 eye of Vertebrates the following points are important — (1) 

 In both cases the eye is situated at the end of a long median 

 tube. (2) In both cases the tube is apparently an evagination 

 of the roof of the brain. (3) In both cases the position of the 

 tube relative to the rest of the fore-brain is the same. (4) In 

 both cases the manner in which the distal end of the tube 

 grows forward away from its point of attachment is the same ; 

 the exception to this manner of growth in some Vertebrates is 

 probably due to the subsequent enlargement of the cerebrum. 

 (5) In Limulus the bulb-like swelling at the distal end of 

 the tube, which gives rise to a pair of median eyes lying 

 close beneath the ectoderm, may be compared with the ter- 

 minal sac of the pineal eye-stalk. (6) The remarkable diver- 

 ticulum of the under wall of this bulb, giving rise to a peculiar 

 whitish body not connected with the surface ectoderm, but 

 which contains undoubted retinal cells, may be compared 

 with a very similar diverticulum in Vertebrates. (7) The 

 proximal end of the eye-stalk in both Limulus and Hateria 

 (Spencer) contains three distinct nerves, two anterior paired 



