1254 Wisconsin Academy of Sciences, Arts, and Letters . 
until a fairly accurate orientation is secured, by moving tbe 
light very slowly to one side as the animal is swimming towards 
it, it may orient itself to a certain extent without this sudden 
movement of the tail. This is accomplished by rotating the 
body so that the ventral surface is more nearly perpendicular 
to the rays; the ventral curvature of the body soon causes its 
course to be deflected towards the light. The changes in the 
direction of swimming brought about by rolling from side to 
side are, however, very slight, and constitute a minor factor in 
the orientation of the body. One eye was cut off in a number 
of individuals but the shock effect prevented any response to 
light and in a few hours all were dead. 
Swimming on the back in Eubranchipus is due in part at 
least to the light. If several individuals are swimming in a 
glass dish which is illuminated from below many of them will 
turn over ventral side downwards and swim close to the bottom 
making efforts to get as close to the light as they can. They 
may be caused to reverse their position repeatedly by moving 
the light above and below the dish. Some specimens, however, 
do not reverse and may not show at any time any marked photo¬ 
tactic response. 
