31 G S. WAT A SB. 



chitinous ball as in Serolis (Fig. 70, PI. XXXV) ; or a cuneiform 

 chitinttns structure, as in Talorcheslia (Fig. 73, PI. XXXV); or 

 finally the whole cell may remain as a clear, transparent body, 

 as in several insects, forming Grenadier's " aconous type " of the 

 compound eye. 



Tiie forms assumed by the rhabdomcrcs in different Arthro- 

 pods are equally diverse. The rhabdomere may exist as a plaiq 

 cuticular covering over the non-pigmen ted part of the retinula, 

 as in Limulus or in Serolis (Fig. 10, Jih, PI. XXIX; Fig. 38, 

 Jib, PI. XXXI, etc.) ; it may become extremely elongated and 

 narrow as in Musca or in Callinecies (Fig. 5, Bh, PI. XXIX; 

 Fig. 37, lib, PI. XXXI; Fig. 72, lib, PI. XXXV); it may 

 become transversely folded as in Cambarus (Fig. 35, lib, PI. 

 XXXI; Fig. 4, Rb, PI. XXIX; Fig. 71, Bb, Ph XXXV); 

 these transverse folds may become still finer, sliowing the 

 chitinous serrature along the axial edge of tlie retinula, as in 

 Penaeus and Ilomarus (Fig. 34, Jib, PL XXXI) ; or this trans- 

 verse serrature may become extremely fine and regular, as in 

 Squilla (Fig. 36, PI. XXXI). 



The cornea undergoes equally diverse modifications according 

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

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

 curvatures and by the varieties of its thickness. In several of 

 the decapod Crustacea which I have examined, as J^enaeus, 

 Cambarus, IJomarus, Callinectes, Gebia, etc., the curvatures of 

 the individual cornea on both surfaces are very slight; it is 

 biconvex in an extremely small degree. In Talorcheslia both 

 surfaces of the cornea are parallel. In Serolis, four species of 

 which I have studied, all having well developed compound eyes, 

 there exists a considerable difference in different species in the 

 nature of the cornea. In some the curvature on the proximal 

 surface is very strong and the whole structure is quite tliick, 

 while in others the cornea is rather thin and a slight development 

 of curvatures exists. This is interesting, showing that even 

 within the group of nearly allied species there are considerable 

 differences in this respect. 



This fact is easy to understand when we remember the func- 

 tional property of the cornea and the crystalline cone. As has 

 been noticed already, the crystalline cone is always dioptric in 



