THE ARTHROPODA. 



417 



tion, recall the appearance of facet-eyes, although a group of eyes does 

 not possess the true structure of the latter. Each ocellus in this way 

 becomes a single ommatidium of the facet-eye. The diminution in 

 number of its elements which it then undergoes, and the simultaneous 

 formation of the rhabdoms are consequences of the subordination 

 and loss of individuality of the originally distinct single eyes on 

 becoming merged in the complex eye, of which organ they now form 

 a part. 



Attempts have been made to trace back the facet-eye to the more 

 primitive form from which it originated, by regarding the ommatidia 

 which, according to the view mentioned above, were derived from 

 single ocelli, as sim- 



C. 



J8. 



pie hypodermal de- (fl 



pressions which, in 

 consequence of the 

 length of the omma- 

 tidia, became very 

 deep (Fig. 197 D). 

 In making such an 

 attempt to explain 

 the structure of the 

 ommatidia it is best 

 to start from a de- 

 pression of the 

 hypodermis which 

 corresponds to a 

 simplified ocellus 

 (Fig. 197 A). As 

 the depression 



deepens and, instead of rods, rhabdoms begin to form in the retinal 

 cells, this eye reaches a grade of development (Fig. 197 B) essentially 

 equivalent to that of an ommatidium in the lateral eyes of Limulus 

 (Fig. 196). The lateral eye of Limulus is composed of a number of 

 single eyes formed of only a few cells (Fig. 196). These unilaminar 

 eyes are quite continuous with the hypodermis, but already show 

 rhabdom- formation (Fig. 196 A, rh). It is indeed not certain 

 whether the eyes of Limulus should really be regarded as primitive 

 eyes, or as degenerate forms of the compound eye ; in any case, 

 however, we can imagine that the higher facet-eyes passed through 

 a similar stage (Fig. 197 B). 



When the depression deepens, another series of hypodermal cells 



2 E 



Fig. 19S.— Diagrams illustrating compound eyes in longi- 

 tudinal section. A, Limulus; B, a larva of Agrion; C, 

 Branchipus (after Watase). The thick black line represents 

 the hypodermis, and each of the depressions formed in it 

 represents an ommatidium. 



