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 Ammocctes 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 the 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 
