FIRST I.HSSONS IN ZOOLOGY 



These are the constant essential parts of an image-form- 

 ing- and image-perceiving eye. In most eyes there are 

 otlier accessory parts which may make the whole eye an 

 ( organ of excessively complicated structure 

 and of remarkably perfect seeing capacity. 

 cc *^*'-n' f>\\n eyes (fig. y6) are organs of ex- 

 treme structural complexity and of high 

 development, although some of the other 

 vertebrates have undoubtedly a keener 

 and more nearly perfected sight. 



"The crustaceans and insects have 

 e)'es of a peculiar character called com- 

 pound e)-cs (figs, jy and 78). In addi- 

 tion most insects have smaller simple 

 e\-es. h~ach of the compound e_\-es is 

 composed of man)' (from a few, as in 

 certain ants, to as man)' as t\\'ent)'-five 

 thousand, as in certain beetles) eye ele- 

 ments, each e)'e element seeing inde- 

 pendentU' of the others and seeing only a 

 very small part of an)- object in front of 



-on 



Fig. 78. — Sccti<.i 



tliroiiLili 



lew 



tac«.-ts ai.il i-\c the whole e)'e. All the small parts of 



elements of" the , ' , . , , 



compouml e\e of the external object seen b)' the many 

 a }iiotli; J. c'lr- (jj^^tinct e)'e elements combine so as to 



neal facets; r.(-, 



crystaliiiieeoijes; form an image in mosaic, that is, made 

 /. P'gnii'it; '-. i^jp q£ separate small parts of the external 



retinal jiarts ;r),;/, ^ ^ ^ 



optic nerve, object. If the head of a dragon-fly be 

 fic"'' iftei'''i':'x' examined it will be seen that two-thirds 

 I'cr.) or more of the whole head is made up 



of the two large compound e)'es, and with a lens it ma)' 

 be seen that the outer surface of each of these e)-es is 

 composed of many small spaces or facets, \\hich are the 

 outer lenses (jf the man)' e\'e elements conijjosing the 

 whole e)'e. 



