Eyes of MoUuscs and Arthropods. 585 



of Pecteii prepared in this mauuer, and the facts thus obtained formed 

 the basis of further observations in other genera of Mollusca, as well as 

 Arthropods. 



Althoiigh the rods of Pecteii bave beeu described by Hensen, 

 Carrière and Hickson, they had a very inadequate notion of their 

 strueture, or even shape, owing- to the lack of proper meaus of prepara- 

 tion. They are eolumnar-shaped bodies of a faint, yellowish-red color, 

 largest near the optic axis and gradually diminishing in size toward 

 the periphery of the retina. They cousist of a hyaline, refractive cap, 

 or sheath, surrouuding a pyramidal, axial core fiUed with a watery, 

 uon-refractive fluid, and, a short distance from the inner ends of the 

 rods, terminating in a rounded apex (PI. 29, fig. 20). 



Each retinophora contains an axial nerve fibre [ax.f.]^ which, 

 entering the attenuated end of the cell, is continued through the first 

 vesicle-like swelling (to whose wall it seems to cling), past the second 

 nucleus, and on, through the centre of the cell, to the inner end of 

 the rod, whence it issues, and, dividing into two main branches, be- 

 comes united by connecting loops with the axial fibres of neigh- 

 boring cells (fig. 20 /. ax.f'^.). In order to study these highly inter- 

 esting conditions , it is necessary to remove the rods »en masse«, and 

 then study their inner ends. From the dark, oval area, representing the 

 apex of the axial cone (PL 29, fig. 22) seen in optical section, arises a 

 large nerve fibre, which issues from one side of the rounded inner ends 

 of the rods, and divides into two principal branches, one of which 

 unites with a brauch from a neighboring rod to form a »nerve loop«, 

 while the other, bending nearly double, passes between the rods, 

 there giving rise to innumerable fibrillae which surround their outer 

 walls. The ends of the rods are also supplied with minute nerve fibrillae 

 arising from the loops, as well as from the other branches (PI. 29, fig. 22). 



Toward the inner ends of the retinophorae, the axial nerve fibre 

 begins to give off radiating fibrillae, which, in the rods, are so numerous 

 as to constitute the greater part of their substance. When perfectly 

 preserved, which is seldom the case, they form an almost solid core of 

 struight, radiating fibres, some of which, at the inner wall of the sheath, 

 become contiuuous with circular fibres , while others are continued on, 

 through the sheath, to the surface of the rod, there uniting with 

 branches from the external nerves arising from the ganglionic layer 

 or from the ganglionic brauch of the optic nerve (PI. 29, figs. 20 and 24). 

 In some preparations, the core seems to be filled with fine granules, 

 produced by the coagulation of the fibrillae. In others, the fibrillae are 



