Eyes of Molluscs and Arthropods. 557 



ed by prolong-ecl confinement. It is, however, not at all diffieult to ob- 

 tain excellent preparatious, in which the nerves now to be described 

 may be easily studied. 



Among the isolated cells of such preparations, one can easily dis- 

 tinguish the retinophorae — either with or without the rods — some- 

 times with the pigmented cover cells attached to them. It is very seldom 

 that one can trace the nerve fibres, or find their broken ends upon the 

 outer row of cover cells, but I have succeeded in doing so several times. 

 In such cases, one sees extremely fine fibres, either running along the 

 stalk of the cell, or projecting considerably beyond its base. Thè fibre 

 is ofteu free everywhere, except at the outer, or pigmented, end of the 

 celi, to which it is attached so firmly, that considerable rapping upon 

 the cover glass is necessary before it can be entirely isolated; and, in- 

 deed, eveu with the roughest treatment, it is sometimes impossible to 

 separate it from the celi. This is due to the factthat lateral nerve fibril- 

 lae. applied to, or penetrating, the celi wall, are only formed at the 

 pigmented portion of the celi, in which the nucleus is situated. 



The nerve fibres of the inner row of pigment cells are larger and 

 more numerous than those of the outer row. To study these nerve 

 fibres, the cells must first be isolated, to do which requires a more pro- 

 longed maceration than is necessary for those of the outer row. Very 

 instructive preparations are obtained by a slight maceration, which not 

 only separates the ommatidia from each other, but removes the outer 

 row of pigment cells, leaving the retinophorae and their rods stili sur- 

 rounded by the inner row of retinulae. The whole group of cells has a 

 conical shape, the rod forming the outermost and widest part (fig. 59) . 

 By further maceration, the inner row of pigment cells is isolated, and 

 one may then distinguish, on each side of the flattened surface, from two 

 to five refractive and colorless lines, which are seen to be distinct fibres, 

 and not cell markings as might easily be supposed from the fact that. 

 in almost every celi, some are either partly detached from, or project 

 beyond, the inner end of the cells (fig. 60). With a good light, and a 

 strong immersion lens, one can see exceedingiy fine, cross fibrillae, which 

 arise, atquite regulär intervals, from the larger, longitudinal nerve fibres: 

 where the latter project beyond the celi, the lateral fibrillae may stili be 

 seen, no longer as straight as before, but either so closely curled, or so 

 strongly varicose, that one would hardly recognize them as fibrillae. In 

 the usuai macerated preparations, these pigment cells extend only about 

 two thirds of the way from the basai membrane towards the outer sur- 

 face, where they appear to cease abruptly. If, however, the cells be iso- 



