CHAPTER II 



THE EVIDENCE OF THE ORGANS OF VISION 



Different kinds of eye. — Simple and compound retinas. —Upright and inverted 

 retinas. — Median eyes. — Median or pineal eyes of Ammocoetes and their 

 optic ganglia. — Comparison with other median eyes. — Lateral eyes of verte- 

 brates compared with lateral eyes of crustaceans. — -Peculiarities of the 

 lateral eye of the lamprey. — Meaning of the optic diverticula. — Evolution 

 of vertebrate eyes. — Summary. 



The Different Kinds of Eye. 



In all animals the, eyes are composed of two parts. 1. A set of 

 special sensory cells called the retina. 2. A dioptric apparatus for 

 the purpose of forming an image on the sensory cells. The simplest 

 eye is formed from a modified patch of the surface-epithelium ; cer- 

 tain of the hypodermal cells, as they are called, elongate, and their 

 cuticular surface becomes bulged to form a simple lens. These 

 elongated cells form the retinal cells, and are connected with the 

 central nervous system by nerve-fibres which constitute an optic 

 nerve ; the cells themselves may contain pigment. 



The more complicated eyes are modifications of this type for the 

 purpose of making both tho retina and the dioptric apparatus more 

 perfect. According to a very prevalent view, these modifications have 

 been brought about by invaginations of the surface- epithelium. 

 Thus if ABCD (Fig. 28) represents a portion of the surface-epithelium, 

 the chitinous cuticle being represented by the dark line, with 

 the hypodermal cells beneath, and if the part C is modified to form 

 an optic sense-plate, then an invagination occurring between A and B 

 will throw the retinal sense-cells with the optic nerve further from 

 the surface, and the layers B and A between the retina and the source 

 of light will be available for the formation of the dioptric apparatus. 

 The lens is now formed from the cuticular surface of A, and the 



