72 WILLIAM PATTEN. 



greater when we recollect that the invagination cavity of the 

 optic ganglion leads from the sides into this space at a level 

 midway between the iufundibulum and the cerebral hemi- 

 spheres, just as the tubular lateral eye nerve of Vertebrates 

 leads into the sides of the third ventricle. The principal 

 difference between the optic cavities in the two cases is that in 

 Vertebrates the canal extends the whole length of the nerve to 

 the lateral eye, while in Limulus it only reaches to the root 

 of the nerve. It must have extended further along the lateral 

 eye nerve in forms more closely related to Vertebrates, other- 

 wise the lateral eye could not have been inverted, as it is now 

 in Vertebrates. 



If this view is correct, then extending the ganglionic inva- 

 gination to the lateral eyes in Limulus ought to give rise to 

 conditions similar to those in the lateral eyes of Vertebrates. 

 This is indeed the case. It is a well-known fact that the 

 lateral eyes of Limulus, Trilobites, and the Merostomata 

 are kidney-shaped, a configuration they must necessarily 

 have in order to distribute the ommatidia economically over 

 the convex surface of the cephalo-thorax. The concave 

 edge of the eye is always directed hsemally, as I have 

 shown, in a purely diagrammatic way, in fig. 24. Now 

 if the invagination of the optic ganglion had progressed 

 a little farther along the nerve the whole eye would have 

 been invaginated, and the ommatseum would then form 

 at the end of a long tube a kidney-shaped retina, but with its 

 concave edge turned in the opposite direction from before. 

 As a kidney-shaped retina would be no longer of any advan- 

 tage it would tend to assume a circular outline ; but owing to 

 the peculiar distribution of the nerve-fibres and the prede- 

 termined method of growth, this could be most economi- 

 cally and advantageously accomplished by bringing the halves 

 of the concave edge together, thus producing a choroid 

 fissure, the position and direction of which would be 

 like that in Vertebrates. In other words, the kidney- 

 shaped retina of Vertebrates is due to the fact that the retinal 

 cells multiply faster on the convex margin than elsewhere. 



