Eyes of Molluscs and Arthropods. 555 



siou, witli a correspoDding- indentation on tlie outer surfacc. This divid- 

 ine line could be distiuguislied from tlie lougitudinal porcs of the rod in 

 two ways, first, by its greater distinctuess, and, seeond, by the fact 

 that, during a revolution of the rod ou its axis through an angle of OO'^, 

 it disappeared, reappearing by a further revolution. This shows that 

 the median line is an optical section of the plane between the com- 

 pouent halves of the rod. Each half of the inner surface of the latter is 

 eonvex, a cross section of the lower end of the rod always giving a more 

 or less circular outline, containing two ovals, — varying in size with the 

 niveau of the section , — separated by a central band. On the other 

 band, a lougitudinal section of the rod may be doubly concave on its 

 inner surface. while, if the plane of the section is turned through an 

 angle of 90°, the result is a simple , concave outline. The rods never 

 become turbid by the action of any of the usuai histological reagents. 

 In sulphuric acid, they expand somewhat at the outer end, thus slightly 

 exaggerating their normal shape. By a prolonged action of chromic acid, 

 they become constricted in the middle , while the outer and inner ends 

 assume about the same width. After a long treatment with haemato- 

 xylin, they stain an intense blue. When studied with an immersion lens, 

 a series of fine, cross lines , whose curvature corresponds with that of 

 the outer surface of the rod , can be seen. These lines, which appear 

 to be composed of minute dots , or dashes , decrease in distinctuess and 

 frequency towards the inner ends of the rods. Other lines which, in 

 this case, seem more like pores in the substance of the rod, begin at the 

 base, where they are quite broad and well defined , and extend nearly 

 parallel with each other toward the outer surface , where they finally 

 disappear fig. 59). 



Each retinophora, with its double rod, is completely surrounded by 

 eight pigmented cells, four forming an inner, and four an outer row. 

 Each one of the four outer cells is laterally flattened , the inner two 

 thirds being reduced to very thin colorless bands, consisting of hardly 

 more than the cell walls, while the outer ends are wide and thick, and 

 filled with a dark brown pigment. The centre-of each thickened end is 

 thinner than the sides, and is closely applied to, and conforms with. 

 the surface of the retinophorae. In sections, therefore, the inner contour 

 of the four pigment cells forms a circular space filled by the retino- 

 phorae, while the outer contour is more or less pentagonal, or hexagonal. 



In the centre of the pigmented ends of these cells, is situated the oval 

 nucleus, which usually causes a protuberance upon that surface of the 

 cell turned away from the retinophora. The outer end of the cell is 



