470 



COMPARATIVE ANATOMY 



CHAP. 



re 



no 



FIG. 328. Section through the ocellus of a young 

 Dytiscus larva (after Grenacher). ct, Chitinous cuticle ; 



constructed, with this distinction, that they are in most cases completely 

 demarcated vesicles, over which and below the lens the hypodermis 



functions as a vitreous 

 body. 



The structure of the 

 compound eyes agrees in 

 general with that of the 

 compound Crustacean eye 

 (cf. p. 353). There is al- 

 ways a bi- convex corneal 

 lens for each single eye. 

 Eueone and Aeone eyes 

 are distinguished, according 

 to whether crystal cones 

 are formed (as in the 



I, cuticular lens ; gk, cells of the vitreous body ; liy, hypo- Crustacea) Or not Acone 

 clermis ; st, rods ; re, retinal cells ; no, optic nerve. / , . ' , ~ , 



eyes are found in the Coleo- 



ptera (excluding the Pentamera), in the Heteroptera, the Tipularidce among 

 the Diptera, and the Dermaptera, In the acone type (e.g. Tipula) each 

 corneal lens is separated from its neighbour by a strongly pigmented 

 zone. Under each lens lies a conical group of 4 crystal cells, whose 

 proximal end is imbedded between two pigment cells. The retinula, 

 consisting of 7 cells (6 marginal cells and 1 axial cell), joins the above. 

 Each of the retinular cells contains in its proximal portion a nucleus, 

 and in its distal portion a rod (rhabdomere). The single eyes are 

 separated from each other by pigment cells. 



The elements of the optic ganglia (ganglion opticum and retinal 

 ganglion) are arranged in a very complicated manner in the Insecta. 



The Ocellus and the Facet Eye. Attempts have been made to connect these two 

 forms of eyes. According to the view most widely approved, the two eyes are to be 

 derived from a primitive eye resembling the single eye (ommatidium) of the acone 

 eye of Tipula. An increase of the elements of this primitive eye led to the 

 formation of the ocellus ; an increase in number of the primitive eyes and their 

 approximation led to the formation of the compound facet eye. For confirmation 

 of this view we are referred to the groups of closely contiguous single eyes o.f the 

 Myriapoda, considered in connection with the compound eye of Scutigera. But it is 

 difficult to reconcile with this view several facts in the ontogeny of the eye, and 

 especially the structure of the middle eye of the Scorpion. Such a scorpion eye on 

 the one hand contains only a single lens, while on the other the retinal elements are 

 grouped into retinulse. 



The last w'ord is certainly not yet spoken with regard to the finer structure and 

 the morphological significance of the Arthropodan eye, and we must here briefly 

 allude to an entirely new view. According to this view the compound eye consists 

 of two layers : (1) of a hypodermis layer which yields the single corneal lenses, and (2) 

 of a subjacent layer of single eyes. The latter is said to be a single layer, the 

 elements of the single eye known as crystal cells, retinular 'cells, and pigment cells 

 running with their processes through its whole thickness (Fig. 329, A}. The 

 rhabdomes and the rhabdomeres forming them are not secreted products of the 

 retinula, but belong, like the crystalline cone, to the crystalline cone cells, which are 



