340 WILLIAM PATTEN. 



The third gaugliouic invagination of Scorpio and Limulus is 

 not yet extensive enough to involve the adjacent eyes, but it is 

 much deeper than in Acilius; and in Limulus it shows a tendency 

 to repeat the condition found in the preceding segment, for 

 the sense-organ on its lateral edge is finally carried toward 

 the median ventral line, and just falls short of being invagi- 

 nated with the ganglion. Hence it is evident that the next 

 step in the extension of the ganglionic invagination would 

 result in the union of the three pairs of invaginations, and the 

 formation of a continuous fold which would finally grow over 

 and enclose the entire fore-brain, which, with the median and 

 lateral eyes and the optic ganglia, would form the walls of a 

 single sac. The result would be a condition practically like 

 that in Vertebrates. If, as in Scorpions, the appearance of 

 the eyes was deferred until the infolding of the brain was 

 completed, and if no secondary optic tubes were formed, as 

 would naturally be the case if the eyes were degenerate, then 

 the eyes would appear as thickenings of the outer wall of the 

 brain-sac, as in Ascidians and Amphioxus. If there was a 

 tendency to develop the eyes in long tubes as in Limulus, and 

 the formation of the tubes postponed until after the brain was 

 enclosed, the eye would appear at the end of a long tube pro- 

 duced apparently by an evagiuation of the outer wall of the 

 brain- sac (pineal eye and lateral eye of most Vertebrates). 

 On our hypothesis it is not surprising such variations should 

 occur, for similar ones are found in Arthropods. For ex- 

 ample, in Scorpions and Limulus the involved eyes do not 

 appear, as such, until long after they have been carried 

 inward, although they ought to appear much earlier if onto- 

 geny gave a complete picture of phylogeny. 



A careful examination of the diagrammatic figures will, I 

 think, make clear what has preceded, and will, no doubt, sug- 

 gest a number of other interesting comparisons which we have 

 not space to consider here. 



The Median Eyes of Arachnids and the Pineal Eye 

 of Vertebrates. — If we have progressed so far on solid 

 ground, it is evident that the pineal eye of Vertebrates must 



