184 PROFESSOR E. RAY LANKESTER AND A. G. BOURNE. 



two kinds according to their position in the ommateum. 

 There are, firstly, those which form the periphery of the 

 ommateum, and are contiguous with the extra-ocular hypo- 

 dermis cells. These are very long columnar cells, which fill in, 

 as it were, the optically valueless circumference of the ommateal 

 capsule. They may be called perineural cells (fig. 2, f). 



The second kind of indifferent cells are placed between the 

 diverging filamentary extremities of the nerve-end cells. They 

 are very small columnar, closely-fitting cells, quite similar in 

 character to the general hypodermis cells. They may be called 

 "interneural cells.^' They are by no means easy to observe, 

 being liable to be destroyed by the action of the acid which 

 is necessary to remove the abundant pigment with which they 

 are charged (see fig. 1). 



Pigment granules appear to be developed in all the cells 

 of the ommateum as well as in the neighbouring hypodermis 

 cells. But it is difficult to make out in this and in all Arthro- 

 pod eyes what precisely is the situation and the limit 

 of the pigment. Before the pigment is removed observation 

 is impossible ; when it is dissolved by acid it diffuses and stains 

 structures previously devoid of pigment. A partial removal 

 of the pigment by means of solvents seems to be the only 

 method which can give any indications on this matter, and 

 even that is unsatisfactory. Pigment granules appear to be 

 very freely developed in the protoplasm of the ordinary hypo- 

 dermis cells and of the indifferent cells (both perineural and 

 interneural) of the ommateum. But in the nerve-end cells 

 the pigment granules are confined to the surface of the cell, 

 leaving the axis transparent. It will be seen subsequently 

 that in the central eyes the nerve-end cells are very nearly if 

 not quite devoid of intrinsic pigment granules, and one is 

 led to question whether the pigment which clothes the nerve- 

 end cells may not in all cases be of external origin. This, 

 however, cannot, it seems, be maintained. We have to admit 

 that the nerve-end cells sometimes produce peripheral pig- 

 ment granules, and sometimes are devoid of pigment. 



The relation of pigment to the optical apparatus cannot be 



