280 bulletin: museum of comparative zoology. 



observed in various crustaceans bj Exuer ('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 gi-anules 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 througli 

 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 subjected 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 crustaceans 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 Palsemonetes, 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- 

 mer case, the pigment lies entirely within the limits of the retinular 

 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 



