84 



ZOOLOGY 



eyes in the animal kingdom. The most familiar of these is the "camera eye" of 

 vertebrates, so called because it illustrates the principles made use of in the 

 construction of the photographic camera. In this there is a lens or body which 

 refracts the rays of light in such a way that all the rays' passing from a point of the 

 object are brought to a focus at a point on the retina (Pig. 43, D). Another type 

 of image-forming eye is the compound eye of insects and Crustacea (Fig. 44). 

 These are made up of a large number of eye elements — each structurally complete 

 in itself — whose separately formed images must nevertheless be joined in order to 

 form a picture. 



Fig. 



44. 



—a/, 



B 



-> 



,__n 



Fig. 44. Diagram illustrating the compound eye of arthropods. A, the whole eye shown in 

 Bection; B, one of the eye-elements (ommatidium) more highly magnified, c, cuticular facets; «p, 

 epidermis; ^ group of cells forming lens-like body; n, optic nerve fibres; o, optic ganglia; p, pigment 

 cells. 



Questions on the figures. — In what way is the independence of each ommatidium 

 secured? In other words what is to prevent the light which comes in obliquely 

 from passing from one ommatidium to another? In what conceivable way is a 

 general image obtained from these various partial views? What groups of animals 

 possess eyes of this sort? Compare the diagram B with the figure of the complete 

 ommatidium of the lobster (Fig. 127). 



The degree to which the color-sense is developed among lower animals is very 

 uncertain. The simplest animals may respond differently to light of different 

 colors, but this is a very different thing from saying that they possess the color- 

 sense in the meaning that we give to the word. 



To summarize, — the essential part of the eye is the sensitive layer known as the 

 retina. The other parts of the complex eye-structure serve the purposes of shutting 

 out the light except from certain directions; of focusing the light admitted in such a 

 way as to increase its intensity and form an image on the retina; of adjusting this 

 apparatus to objects at different distances; of nourishing and supporting the more 

 important portions of the apparatus; and of moving the eye so as to take into view 

 different portions of the surroundings. Some of the various grades of complexity 

 of eye-structure in the invertebrate series beginning with a pigment spot and 

 ending with a complete lens-eye, are shown in Fig. 43. 



