154 THE EYES OF ARTHROPODS. 



eliminate the lens vesicle from the category of organs foreign to the eye. It may 

 be comparable with the thick-walled ectodermic cup that secretes the chitenous 

 lenses for the parietal eyes of arthropods. (Fig. 108.) When the chitenous 

 exoskeleton atrophied, the old lens cup probably remained to form the new lens 

 of the vertebrate eye. 



Origin of the "Imperfections" in the Vertebrate Eye. Thus the com- 

 pound eye of arachnids, which lies on the outer surface of the head, is represented 

 in vertebrates by the retinal placode which there lies in the walls of the brain 

 chamber. The vertebrate retinal placode still retains essentially the same con- 

 tour, surface curvature, and arrangement of visual cells and nerve cells as that of 

 its arachnid prototype, but owing to the inversion of the placode, which took place 

 when it was transferred to the brain chamber, the concave surface and the con- 

 cave margin of the retinal placode face in nearly opposite directions in vertebrates 

 from what they do in the arachnids. 



Thus arose those extraordinary imperfections of the vertebrate eye, which 

 have so often excited the comments of the physicist, anatomist, and philosopher. 

 The inverted retina, the choroid fissure, and the blind spot caused by the awkward 

 entrance of the optic nerve, are the inevitable result of a combination of condi- 

 tions, some of which, originally, had no relation whatever to the eye. These con- 

 ditions were established in the arthropods long before the vertebrate stock ap- 

 peared, and it was a purely incidental, or accidental, result of these conditions that 

 the eye was swept into the brain chamber, where it did not originally belong. In 

 other words, the fate of the lateral eye was not decided by what was best for the 

 eye, as an instrument, or by any selective action, in which the eye itself played 

 a part. The eye was a purely passive victim of its location, and of its more power- 

 ful neighbor, the brain. But it survived, in spite of its unfortunate location, al- 

 though it will forever bear the marks of a displaced and made over organ. 



III. THE OPTIC GANGLIA. 



In reconstructing the history of the vertebrate brain, the structure and posi- 

 tion of the optic ganglia of arthropods is no less significant than that of the eyes. 



Location. We have already shown that in the embryos of Acilius the optic 

 ganglia consist of three lobes lying on the lateral margins of the neural plate, each 

 lobe lying opposite a forebrain neuromere. (Fig. 14.) 



When the larval ocelli of insects degenerate, the ocellar ganglia, without 

 noticeable transformation, become the ganglia of the lateral eyes. But in Limulus 

 and in the arachnids generally, the ganglia of the parietal eye and those of the 

 lateral eyes are separate. In most insects and "Crustacea, the ganglia retain their 

 lateral position through life. This is also the condition in young Limuli, but later 

 the great overlapping lobes of the hemispheres crowd the lateral eye ganglia to- 

 ward the haemal surface. (Figs. 36-39.) Limulus is the only arthropod, to my 

 knowledge, in which the ganglia occupy this position. 



