334 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
tions of its function prior to the knowledge of the photokinetic phe- 
nomenon, has discussed the modern theories about it, and has added 
new light on the subject from his own investigations. 
Hesse’s anatomical study of the optic organs of many invertebrates 
yielded two facts bearing upon the present problem: — (1) in certain 
leeches and annelids are found unpigmented cells which, because of 
their resemblance to pigmented cells or eye-spots in close relatives, 
are undoubtedly visual cells; (2) often where pigment is present it is 
either separated from the sensory cells by an intervening tapetum, 
as in Pecten and some insects, or it is diversely situated in eyes of 
closely related species, as among the gastropods. From this it is 
seen that pigment is not essential to the physiological reception of 
light nor —as the experiments of Harrington and Leaming (:00) 
and of Mast (:11) on Amoeba indicate,— even of color; and although 
pigment be present, yet its loose association with the visual cell bars. 
attributing to it a primary réle. When the relation between the two 
is closer, the pigment-cup, by partially screening the cell, enables the 
animal to determine the direction whence the light comes. 
The phenomenon of migration of the pigment probably arose with 
the evolution of the eidoscopic eye, with which, at first, only moving 
objects, and later stationary forms as such, were perceived by the 
animal. At the outset, before the significance of the pigment can be 
logically discussed, an answer must be sought to the fundamental 
question: — where are the receptive organs in the eye of the crayfish 
located? 
Parker (’95) has emphasized two essential features which the 
receptive organ must have:— first, a dioptric mechanism for trans- 
mitting the light to it; and, secondly, nerves for conducting the 
stimulus to the brain. The phenomenon of the pigment-migration 
may aid in identifying the receptive region. Thus, Hesse (:00) has 
questioned Grenacher’s conclusion that the rhabdome in the retina 
of the cephalopod is the sensory organ, and. has pointed to another 
structure as being more probably the sensory organ, because of its. 
closer relation to the emigrated pigment. In the crayfish the optical, 
neurological, and photokinetic evidence — for a discussion of which 
see Parker (’95) and Hesse (:01) — points to the rhabdome as the 
region in question. As to the vertebrate eye, the evidence of neuro- 
fibrillae on the outer segments of the rods and cones furnished by 
Hesse (:04) and Howard (:08); the observations by Van Genderen 
Stort (’87), Chiarini (:04 and :06), and Herzog (:05) of the pigment- 
migration extending only out to the ellipsoids of the rods; the dis- 
