REACTIONS TO LIGHT AND PHOTO-RECEPTORS OF ANNELIDA. 
WALTER N. HESS. 
As one surveys the more important groups of Annelida, he at 
once discovers that these groups differ considerably from one another 
in their possession of eyes or other cells which function as photo-re- 
ceptors. Although this paper will refer briefly to Nereis virens as rep- 
resentative of the Polychaeta, and to Glossiphonia parasitica, one of the 
Hirudinea, it will be chiefly confined to a discussion of our common 
member of the Oligochaeta, Lwmbricus terrestris. 
Nereis virens, as is well known, possesses two pairs of eyes on 
its prostomium. If the worms of this species are exposed to lateral 
illumination, from either the right or left side, they react negatively 
and orientate readily in a negative direction. If the eyes of a normal 
specimen are removed from one side with a sharp scalpel it no longer 
reacts as before, but produces “circus movements” in that it turns 
chiefly away from the side possessing the functional eyes. With all 
four eyes removed no reactions to light are apparent. From these 
experiments we feel warranted in stating that the cells which function 
in photo-reception in Nereis virens are definitely localized, and are 
probably found nowhere else except in its four eyes. 
Our common leeches, such as Glossiphonia parasitica, possess sev- 
eral paired segmental eyes which function as photo-receptors in a 
similar manner as the eyes of Neveis. Here the eyes are more numerous 
and are more widely distributed, however, than in Nereis. 
Our common earthworm, Lumbricus terrestris, possesses no per- 
ceptible eyes, yet it responds readily to the effects of light stimulation. 
Normal worms of this species, when exposed to light of ordinary in- 
tensities readily move away from the source of illumination, and orien- 
tate very definitely in a negative direction. These same worms, which 
are negative to light of ordinary intensities, become positive, in keeping 
with their nocturnal habits, when the light is greatly diminished. If 
the brain of a normal earthworm is removed by a dorsal incision, or 
by the removal of the first three anterior segments, the worm no longer 
reacts negatively to ordinary illumination, but it becomes strongly posi- 
tive, and if six, or even more of the anterior segments are removed, they 
are still positive. There is a brief period, however, of only a few 
seconds’ duration when first exposed to light that these worms give 
negative reactions. In each case they quickly adapt themselves to the 
light and become positive. Similar results were obtained with Allolo- 
bophora foetida with as many as forty anterior segments removed. 
These results show that the brain of the earthworm is not necessary 
for reaction to light and orientation. They, however, indicate that 
earthworms are more sensitive to light when the brain and the photo- 
receptors at the anterior end are functional, than they are when these 
are not functional. This accounts for the fact that, while normal 
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