78 ZOOLOGY. 



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 



FIG. 42. 



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FIG. .42 Diagram illustrating the compound eye of arthropods. A, the whole eye 

 shown in section; B, one of the eye-elements (ommatidium) more highly magnified, c, 

 cuticular facets; ep, epidermis; /, group of cells forming lens-like body; n, optic nerve 

 fibres; o, optic ganglia; p, pigment cells. 



Questions on the figure. 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 t the complete ommatidium of the lobster 

 (Fig. 125). 



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 

 compexity of eye-structure in the Invertebrate series beginning with a 

 pigment spot and ending with a complete lens-eye, are shown in Fig. 41. 



no. Analogy and Homology. In comparing various 

 animals we find that they may do the same work with organs 

 that arise in very different ways, which however, because they 

 are adapted to perform similar tasks, look somewhat alike. 



