836 THE AMERICAN NATURALIST. [Vow. XXXII. 
sensory cells contain fibrils that may act as conductors or 
as resonators to certain kinds of movements. _ 
The following is a very brief statement of the most impor- 
tant conclusions we have reached concerning the structure and 
arrangement of retinal cells. 
(1) Structure of a Retinal Cell.— A typical retinal cell or 
retinophora, such as those found in molluscs and arthropods, 
although it has the outward appearance of an ordinary sensory 
cell, is in reality a double, or twin, cell. It consists of two 
nearly equal parts, readily distinguished by differences in 
optical properties; each part contains a nucleus and supports 
half of the rod (Fig. 11). A large nerve fibre is attached to 
the base of the cell and there breaks up into a dense mass 
of fibrils that penetrate and surround the cell and the rod. 
Although there is no sharp demarcation between them, we are 
able to distinguish three sets of these fibrils. The first set 
consists of comparatively coarse fibrils running lengthwise of 
each retinophora between its two parts; they are often closely 
united to form a distinct axial bundle extending into the rod 
and sometimes beyond its outer end. The second set rami- 
fies over the surface of the cells and the rods. Some of them 
appear to be independent intercellular nerve fibres not united 
with the large bundle attached to the base of the cell. The 
third set is found mainly in the rods, where they form what I 
have called a retinidium, which consists of a series of extremely 
delicate transverse fibrils that appear to unite the coarser axial 
and the superficial rod fibres with one another. These cross 
fibrils give the rods either a finely dotted or a striated appear- 
ance, according to the direction from which one looks at the 
fibrils. 
Such a retinal cell, therefore, appears to contain a mass of 
fibrils which extend through the center of the cell into the 
rods, where they bend at right angles and become continuous 
with those on the peripheral layers. At the base of the cell all 
the fibrils are gathered together to form an axis cylinder, and 
after extending some distance, they separate and end freely. 
(2) Position of the Visual Rods. — The rods may be upright, 
inverted, or, very rarely, horizontal, as in the ocelli of some 
