MORPHOLOGY OF COMPOUND EYES OF ARTHROPODS. 147 



The distal ends of such ( V.) which were thus pushed over will 

 meet one another in the median or the " optic axis " of the 

 ommatidium; further, they will continue to secrete chitin 

 (fig. 3j c.c.) from their original chitin-secreting surfaces which 

 are now median and axial. The chitin thus secreted will 

 have an independent existence from the cornea, thus 

 forming the rudiment of the crystalline cone, and the cells 

 themselves will form the vitrellse (figs. 3, 4, V., &c,). Finally 

 as the deepening still further goes on, the corneal lens(C.) and 

 crystalline cone (c. c.) will be entirely separated, thus producing 

 a condition somewhat similar to that which obtains in Scrolls 

 (figs. 5 and 6). 



From this point onward, the three chitinous structures 

 cornea, crystalline cone, and rhabdomere, undergo a 

 difi'erent development in different Arthropods. In some the 

 crystalline cone assumes a transparent semi-liquid state, while 

 the whole cell becomes extremely elongated^ forming the 

 crystalline cone of certain Crustacea (PI. XIX, fig. 7) ; it 

 may form a hard chitinous ball as in Scrolls (PI. XIX, 

 fig. 6); or a cuneiform chitinous structure, as in Talo- 

 chestia (PL XIX, fig. 9); or, finally, the whole cell may 

 remain as a clear, transparent body, as in several insects, form- 

 ing Grenacher's " aconous type " of the compound eye. 



The forms assumed by the rhabdomeres in different Arthro- 

 pods are equally diverse. The rhabdomere may exist as a 

 plain cuticular covering over the non-pigmented part of the 

 retinula^ as in Limulus or in Scrolls; it may become 

 extremely elongated and narrow as in Musca or in Calli- 

 nectes (PI. XIX, fig. 8, Rb.); it may become transversely 

 folded as in Cam bar us (PI. XIX, fig. 7, Rb.) ; these transverse 

 folds may become still finer, showing the chitinous serrature 

 along the axial edge of the retinula, as in Penaeus and 

 Homarus; or this transverse serrature may become ex- 

 tremely fine and regular, as in S qui 11a. 



The cornea undergoes equally diverse modifications accord- 

 ing as it is purely protective, or partly protective and partly 

 dioptric in function. The range of variation is shown by the 



