112 



EVOLUTION AND GENETICS 



Surface of lens 



Cornea \ 

 Iris 



Lens. 



Retina ' 



Optic ganglion 



Anterior 

 • optic cliamber 

 Cornea 



s. Posterior 

 optic chamber 



Eyes. In the eyes of Arthropoda, Mollusca and vertebrates we 

 find a remarkable example of this kind. All are special sense organs 

 for the reception of light stimuli, and in their highest development, 

 for the formation of visual images, yet they are very different 

 structures. 



Insect eyes are of two types, simple and compound; the former 

 may be composed of numerous visual cells grouped beneath a 

 transparent lenticular cornea, developed from the hypodermis 

 (Fig. 62). In the compound eye similar visual cells form units with 

 accessory cells and a separate cornea. These units are called om- 



matidia (Fig. 63) and are associ- 

 ated in large numbers in the most 

 highly developed eyes. Their 

 action is explained by Miiller's 

 theory of mosaic vision. Accord- 

 ing to this theory each omma- 

 tidium records a point of light, 

 not a complete image. The result 

 of numerous points of light re- 

 FiG. 64. — Diagrammatic section of corded by reflection of rays from 

 the eye of a squid, Loligo. (From different parts of an object is an 

 Heener, after Grenadier.) , . . ™i • • 



erect mosaic miage. Ihis rniage 



would depend for its resemblance to the original on the numljer of 

 ommatidia in the eye, and the resulting completeness of repro- 

 duction of details. 



Eyes of Molluscs and Vertebrates. Molluscan eyes as de- 

 veloped in the Cephalopoda and vertel^rate eyes are very different 

 in optical function from the insect eye. Each is provided with a 

 lens whjch forms on the retina a complete image of any object 

 within the field of vision. This is, of course, an inverted image. 

 In visual function the two eyes are similar. In structure and 

 origin, however, they are different (Figs. 64 and 65). 



Both cephalopod and vertebrate eyes have an outer cornea, 

 behind which is an anterior space or chamber. Between this and 

 a larger posterior chamber lies the lens. In front of the lens the 

 iris governs the size and shape of the pupil, and at the back of 

 the posterior chamber the light-sensitive retina is located. In the 

 vertebrates, however, the posterior chamber of the eye, and con- 

 sequently the retina, are derived from the first brain vesicle, while 

 in the Cephalopoda they develop directly from the outer ecto- 



