580 William Patten 



he has describecl, really are. It has not, in any case, been proved, that 

 simple, pigmeuted cells are sufficiently sensitive to changes in the 

 amount of light to cause muscular contractions (the only evidence that 

 we at present possess to show that such a fuuction is present) ; on the 

 other hand, it is extremely probable, from my observations, that the 

 irritability to light is only found, toaperceptibledegree, in what I 

 have called the isolated ommatidia. 



The much deeper pigment of the iris on the side toward the light, 

 and the absence of nerve fibres, or isolated ommatidia, indicate that the 

 iris is merely a structure for the exclusion of lateral rays of light from 

 the retina. 



The cilia, which Hensen believed to have seen on the outer ends 

 of these cells, were probably ragged ends, caused by maceration or 

 tearing away of the cuticula, or they belonged to ciliated cells from 

 some other part of the mantle, since no cilia, or sense hairs are to be 

 found on any part of the epithelium of the eye. 



The stalk itself consists of loose, connective tissue, often containing 

 enormous bloodspaces. Besides the connective tissue forming the w^all 

 of the lacunae, there is an upper and lower group of long, striated, 

 muscular cells, serving as erectors and depressors of the eye. They are 

 continued from the stalk toward the anterior pole of the eye, into the 

 connective tissue layer (PI. 29, fìg. 19 e. t. e), where, the cross Striae 

 disappearing , they are replaced by numerous, fine, smooth fibres, 

 interspersed with nuclei. The fibres decrease in size and distinctness, 

 until, just beneath the cornea, they form an almost structureless and 

 hyaline layer — the pseudo-cornea, — in which nuclei are seldom 

 found. Beneath the iris, some of the fibres are seen to originate from the 

 ends of pigment cells, while, atthe edge, many fibres which, up tothat 

 point were distinctly visible, appear to terminate with an out ward curve, 

 as though attached to the epithelium at that point, forming what we 

 shall cali the ciliaris (PI. 29, fig. 19 e. L). 



The lens which, as the development shows, consists of a modified 

 group of mesodermic cells, continuous with those of the pseudo-cornea 

 and connective tissue capsule, is round and biconvex, with the inner sur- 

 face much more curved than the outer. It consists of large, irregulär, 

 granular cells with very distinct walls and excentric nuclei. The elon- 

 gated cells in the middle of the lens are the largest, the long diameter 

 being parallel with the optic axis. Toward the inner surface, they be- 

 come flattened and strap-shaped to form cells, the nuclei of which seem, 

 in many cases, to have entirely disappeared. On the anterior side, the 



