ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 345 



ones and a totally different unpaired one. (8) In both 

 cases the unpaired nerve may at certain stages be a mere 

 hollow tubCj the Avail of which is composed of ordinary 

 columnar cells, showing no trace of nerve-structure. (9) 

 In both cases the proximal end of the eye-tube may re- 

 present either the last point where the brain is connected 

 with the ectoderm (Leydig), or the lips of a round, median 

 opening or ueuropore, leading into the cavity of the pineal 

 eye or into the brain, or into both, according to the stage 

 and method of development. (10) The manner in which, in 

 Vertebrates, the fore-brain is enclosed and the median eye- 

 tube formed represents a modification of a fundamentally 

 similar process found in Limulus, and the Arachnids generally ; 

 and this fact is of the utmost importance on account of the 

 peculiar and complicated nature of the process, and its total 

 absence in other groups of animals. 



The lateral eyes of Scorpion and Limulus are not in- 

 volved by ganglionic infoldings so as to lie at the ends of brain- 

 tubes, but, as shown above, the next step in the changes 

 already accomplished would probably lead to that condition. 

 Suppose a kidney-shaped eye, such as is usually present in 

 and best adapted to forms like Limulus and other Merosto- 

 mata, came to lie at the end of an optic tube; then a kidney- 

 shaped retina would be produced with its concave edge 

 directed haemally, instead of in the opposite direction, as in 

 Limulus, &c. As a kidney-shaped retina would no longer be 

 required, it would naturally become circular ; and, owing to 

 the peculiar distribution of nerve-fibres, this would be most 

 easily and economically accomplished by bringing the halves 

 of the concave edge together, thus producing a choroid fissure, 

 the direction and position of which would be like that in 

 Vertebrates. 



Now the oramateum of Limulus consists of circles of from 

 fifteen to twenty retinal cells surrounding a single central 

 one of a little different character (Watasi). If such an omma- 

 teum were converted into a true retina the arrangement 

 of the cells would probably be retained, and we would have a 



