418 GENERAL CONSIDERATIONS ON 



may be drawn into the formation of the eye (Fig. 197 C), these 

 representing the crystalline cone-cells (kz) of the ommatidium. A 

 series of lentigen cells may also be utilised in the formation of the 

 eye (Fig. 197 C, l.g). The further deepening of the optic pit, and 

 the great lengthening of the cells lead finally to the form of the 

 ommatidium (Fig. 197 D). The hypodermal cells, the lentigen cells, 

 the crystalline cone-cells, and the retinal cells thus appear as a uni- 

 larninar layer of long cells penetrating far down, and having the same 

 arrangement as in the simple ocellus (Fig. 195). The lumen here, 

 however, is not open as in the ocellus, but filled by the mass of the 

 crystalline cone and rhabdoms, but this does not constitute an 

 essential difference between the two eyes. The grouping together in 

 larger or smaller numbers of these single eyes which arise as simple 

 depressions of the hypodermis is elucidated by Fig. 198, which at the 

 same time represents the arrangement of the ommatidia on a convex 

 base usual in most facet-eyes, and determined by the functional re- 

 quirements of the eye. 



The method of composition of the facet-ej'e here described is essentially in 

 keeping with the view long ago maintained by Gkenachek. This author 

 starts from a simple eye consisting of few elements, such as is represented by 

 an ommatidium of an acone facet-eye of the Tipulidae, and derives the facet- 

 eye through the increase in number of these eyes, and the ocellus through the 

 multiplication of the elements with the retention of the single lens. In the 

 simple eye, which here forms the starting-point, we have an ocellus of specially 

 simple structure. 



It has already been stated that the compound eye of the Crustacea 

 must be regarded as belonging to another ontogenetic series. It will 

 therefore not be a matter of surprise to find that it deviates in many 

 ways from the above in its development. The character of the 

 compound eye is, in the Crustacea, always preserved. In some cases, 

 e.g., in the Isopoda, it might appear as if we had before us transition 

 stages between the simple and the compound eye, but it is more 

 than probable that, in this branch of the Crustacea, we have to do 

 merely with a simplified form of the facet-eye. 



This view of the Isopodan eye was adopted long ago by Gkenachek, who 

 attempted to solve the question as to how the very simple eye of the Isopoda 

 was related to other Arthropodan eyes, by maintaining that the former was 

 to be regarded as a compound eye in consequence of its possessing a double 

 crystalline cone and a retinula forming rhabdoms and divided into seven parts. 

 It cannot therefore be doubted that, in the Isopodan eye, which is not unlike 

 a group of single eyes, we have a secondary form, and this is in itself very 

 probable, inasmuch as the Isopoda are, in many respects, a highly modified 

 group of the Crustacea. A degeneration of the facet-eye, which was originally 

 stalked in the Malacostraca, has taken place in any case in this order. 



