280 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
observed in various crustaceans by Exner (91), Szczawinska (91), and 
myself (Parker, 95); and, so far as the chief facts of these changes 
are concerned, the accounts given by these writers are in substantial 
agreement. In no case, however, has the precise character of these 
changes been followed, nor the time needed for their completion been 
recorded, 
In a proximal retinular cell that shows the full effect of light (Fig. 1), 
the black pigment granules are almost uniformly scattered from the 
distal end of the cell backward through its whole length, including the 
retinal nerve fibre, to the region of the first optic ganglion. In the body 
of the cell proper (Fig. 6), as well as in the retinal nerve fibre (Fig. 7), 
it will be observed that the pigment granules lie entirely within the 
limits of these structures; in other words, the black pigment of this 
portion of the retina is contained entirely within the proximal retinular 
cells. "This pigment, though in the main uniformly distributed through 
the cell, shows regularly two slight concentrations, — one at the swollen 
distal end of the cell (Fig. 1), and another on the sides of the rhabdome. 
Small irregular concentrations may also occur in the body of the cell. 
In the eye subjocted to light, the only part of the cell except the nu- 
cleus that is entirely free from pigment granules is a transparent axis 
that can be traced from the region of the rhabdome down through the 
‘body of the cell, and through the whole length of the retinal nerve 
fibre. This is undoubtedly the axis cylinder of the nerve fibre, which, in 
its passage to the rhabdome, extends through the body of the cell. 
In erustaceans like Cancer (Parker, '91, p. 116, Plate X. Fig. 131), 
in which the proximal retinular cells are more fully provided with pig- 
ment granules than in Palemonetes, this axis is more conspicuous. 
In an eye that has been kept in the dark for several hours, the bodies 
of the proximal retinular cells are without trace of pigment (Fig. 2), 
the whole mass of black pigment being concentrated in the retinal nerve 
fibres, i. e. proximal to the basement membrane. Here, as in the for- 
mor case, the pigment lies entirely within the limits of the retiuular 
cell. 
The transition from the dark condition to the light condition of the 
eye was accomplished by the following steps. In an eye that had been 
kept some four hours in the dark and then exposed for five minutes to 
the light, the arrangement of the pigment in the proximal retinular 
cells was indistinguishable from that characteristic for full darkness. 
After ten minutes’ exposure to light, the pigment was found to have 
moved forwards to the level of the basement membrane. After fifteen 
