Eyes of Mollusca and Arthropods. 627 



filledwith granular protoplasm, arevisible by contrast with the structure- 

 less, underlyiug- cuticula. The divisiou between each pair of cells is 

 only indicated by a very faint line, vvhile the cells belonging to ueigh- 

 boring facets are separated from each other by a clear , narrow 

 spaee. In the centre of each facet, is a dark spot, or Impression, from 

 which a fine chitinous-like fibre often projects (tìg. 75 x). One might 

 easily overlook the nuclei of the cuticular cells . as they do not absorb 

 coloring fluids to any great extent, neither have they a high index 

 of refraction. They are usually ovai, and perfectly homogeneous, often 

 showing no trace whatever of that granular structure so characteristic 

 of nuclei in general ; but occasionally they are granular , and stain so 

 deeply that there can be no doubt of their nuclear nature. By boiling 

 a piece of the cuticula a short time in caustic potash, ali the less resis- 

 tant parts are dissolved , and the corneal facets alone left intact , their 

 dark boundary lines being widened at the corners to form figures like 

 those represented in the piate (fig. 75 d.f.) . In the centre of each facet, is 

 a small , round Impression , usually the most striking one , while in the 

 middle of each side, are similar markings looking like the ends of fine 

 fibres , or the impressions where fibres had been attached. A series of 

 extraordinarily faint, parallel lìnes, coinciding with the divisionsbetvreen 

 two corneal cells, separates each facet into halves. The impressions 

 (fig. 75 y) indicate where the outer ends of the ommatidial cells were 

 attached to the corneal membrane, which has at those points fused 

 with the cuticula, upon which impressions of the ommatidial cells are 

 thus produced , or fragments of their outer ends left hanging. When 

 the treatment with caustic potash has beencarried to excess, ali mark- 

 ings disappear, except the contours of the facets. 



Beneath the corneal celi layer is the infinitely thicker, ommateal 

 hypodermis, composed of numerous ommatidia corresponding in 

 number with the corneal facets. Each ommatidium consists of a 

 conical group of 19 or 20 very long cells, each one of which 

 extends from the corneal hypodermis to the basai mem- 

 brane. They are arranged in four circles around a centrai axis, 

 ali the nuclei of each group being placed at the same niveau, in 

 speeially enlarged or pigmented portions, thus forming as many zones, 

 at varying heights. as there are circles of cells. The innermost 

 group consists of four colorless cells — the retinophorae — united 

 to form au inverted pyramid, whose base abuts against the corneal 

 hypodermis, while the apex rests upon the basai membrane. The 

 bases of the pyramids are Square , but , passing inward , the corners 



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