Eyes of Molluscs and Arthropods. 579 



siiddenly reduced in thickness and, losing its pigment, becomes trans- 

 formed into the transparent cornea, tlie central part of which is the 

 thickest. 



The cor ne al e e 11s (PI. 29, fig. 31) are columnar, constricted in 

 the middle, and slightly expauded at either end ; the outer one is capped 

 with a ìayer of cuticula, while the wall of the inner is thrown into a 

 uumber of uarrow folds, which fit into corresponding indentations of the 

 neighboring cells. These folds extend nearly to the middle of the celi, 

 and end about opposite the ovai nucleus. An excellent method of study- 

 ing these structiires is either to reraove the cornea as a whole by harden- 

 ing and subsequent maceration, or to isolate the individuai cells. In 

 the former case (PI. 29, fig. 37) , wheu viewed from the inner side, the 

 round serrated ends of the cells may be easily seen, while, by focusing 

 more deeply, the nuclei, and finally the hexagonal or pentagonal outer 

 ends of the cells, appear. In the latter case (by maceration) , besides 

 the inner folds óf the cells, there will be seen, in the median constricted 

 part, a number of irregulär, horizontal teeth (PI. 29, figs. 31 and 19). 

 They are probably modified, longitudinal folds, which, at tliis point, 

 are broken up into irregulär, spine-like projectious, interlocking with 

 those of the adjacent cells. The inner ends of the longitudinal folds are 

 continued inwards as fine fibres, crossing the pseudo-cornea, and unit- 

 ing with the outer surface of the lens (PI. 29, fig. 19). This inter- 

 locking of the corneal cells, which is also found — but in a less perfected 

 form — in the hypodermis of the eye stalk, gives greater firmness and 

 flexibility to the cornea, a condition necessary for the peculiar move- 

 ments produced by the ciliary, and other contractile fibres, which we 

 will describe more fully hereafter. 



The cellsofth eiris only differ from those of the cornea, in their 

 greater size, and in being completely fiUed with pigment. When isol- 

 ated, they seem to possess the same plicated inner ends, as those of the 

 cornea, except that the folds assume more the character of fibres con- 

 tinued into the subjacent connective tissue. The outer ends are capped 

 with a rather thick cuticula. Hensen (12) seemed inclined to regard them 

 as sense-cells, which had something to do with the optic function of the 

 eye. Sharp (18) held a similar opinion, ascribing to them the power of 

 distinguishing light from darkness, just as do the cells in the pigmented 

 spots and grooves in the mantle and sipho of Venus, Mactra and Solen, 

 while, to the complicated organ within, — the eye, — he would attri- 

 buto another function thau that of vision. Sharp's supposition arises from 

 a mistaken notion of what the visual elements, in the pigmented areas 



39* 



