THE HIGHER ORGANISMS 161 



ment cells with subsequent nervous elements in groups. 

 The images gathered by such eyes may be regarded as a 

 kind of mosaic made up of many small bits. There 

 can be no accommodation and no perspective. From 

 such eyes great bundles of nerve fibres pass to the 

 optic lobes of the brain, so increasing its complexity. 

 Among certain mollusks, such as the cephalopods, the 

 eye forms a beautiful and striking organ superficially 



aperture 



FIG. 62. Section through the otocyst of arenicola. (After Ashworth and 

 Gamble.) 



resembling the vertebrate eye, but having certain essen- 

 tial differences, for the retinal nerve cells are directed 

 toward the centre of the globe and are outside of the 

 pigment layer, while in the more perfect vertebrate 

 organ the nerve endings are directed away from the 

 centre and the pigment layer of the retina is outside. 



As the structure of the eye increases in perfection the 

 number of nervous elements increases greatly, and the 

 complexity of the central nervous system is increased 

 both by the increased number of fibres it receives and 

 the number of cells with which they communicate, so 

 that the new centres, optic lobes, and optic thalami make 

 their appearance. 



Hearing. In this sense we doubtless have to do with 

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