CO.ypOUND KVES OF ARTHROPODS. 315 



a part of their surface. Assuming the surface where the chitin 

 is secreted to be tiio exterior, we may describe tlio ommatidium 

 of the lateral eye of Limnlus as a group of modified ectodcrmic 

 cells aggregated around and beneath the funnel-sliapod depres- 

 sion in the skin. A glance at the diagrams (Fig. G, PI. XXIX ; 

 figs. 65 and C6, PI. XXXV) will show this clearly. The sensory 

 cells of the ommatidium come into direct contact with tlie conical 

 lens, which is the thickened part of the general cuticle; or, to 

 express this in the phraseology of Lankester and Bourne, the 

 ommatidium in the lateral eye of ZimwZws is " epistatic." The 

 cornea and crystalline cone as such have no separate existence in 

 this stage. 



Suppose such an onimalidium to become duplicated until a 

 considerable number be formed, as we may safely imagine 

 to have been the case, from the general tendency in the perfec- 

 tion of a visual organ. What will be the result^ The first efi"ect 

 of such an increase of the number of ommatidia in a given area 

 will be the lengthening of each unit in the direction of the 

 ommatidial axis, and the cells ( V, Fig. 67, PI. XXXV) which 

 were situated directly on the outside of the retinulae will travel 

 over and above the sensory portion {Rt and O, Fig. 67). The 

 distal ends of such cells {Y) 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 {c.c, 

 Fig. 67) 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 rudi- 

 ment of the crystalline cone, and the cells themselves will form 

 the vitrellae ( V, Figs. 67, 68, etc.). Finally, as the deepening 

 still further goes on, the corneal lens (C) aTid crystalline cone 

 ((•.<:■) will be entirely separated, thus producing a condition 

 somewhat similar to that \\:iiich obtains in Serolis (Figs. 69 

 and 70.) 



From this point onward, the three chitinous structures, cornea, 

 crystalline cone, and rhahdinncre, undergo a difi'erent develop- 

 ment in different Arthroj)ods. 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 (Fig. 71, PI. XXXV); it may form a hard 



