PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 233 
marked reduction in the angle of compensation, a decrease of about 5°, 
as compared with normal animals. This shows clearly the extent to 
which vision enters into the orientation of these animals. 
c. Both Otocysts removed. The angle of compensation is here reduced 
to 3° and 5°, respectively, for the eyestalks on the side toward and from 
which the rotation takes place. Even without rotation the positions in 
which the eyes are held are not definite, as they are in animals which 
possess otocysts. The stalks often make an angle of 40° or more with 
the vertical, and their movements are no longer correlated. This, 
together with the marked decrease in the angle of compensation, as 
compared with that of blinded animals, makes it evident that in 
equilibration and orientation the otocyst plays a much more impor- 
tant part than does the organ of vision. 
d. Both Eyes blinded and both Otocysts removed. On rotation it was 
found that the compensatory movements of the eyestalks were practically 
wanting. ‘Two individuals only out of fifty showed movements of from 
3° to 5°. In the greater number of cases no movement could be de- 
tected, and in the remainder the angle averaged lessthan 1°. There was 
a still greater tendency for the eyestalks to be held in indefinite positions 
when at rest, and at unequal angles. Fifteen such individuals were kept 
in an aquarium more than twenty days, and after this lapse of time 
practically the same results were obtained, showing that the shock of the 
operation of removing the otocyst had no effect upon the results of the 
experiments. Furthermore, removal of the antennules distal to the oto- 
cysts had absolutely no inhibiting effect upon the movements of the 
eyestalks. 
This series of experiments corroborates, as far as they go, the conclu- 
sions of Clark (96). It is clear from them that the otocyst is the chief 
organ in equilibration, though sight also plays an important part in the 
orientation of these animals. 
Since the above work was done (July, 1899) a paper has been pub- 
lished by Lyon (99) on the comparative physiology of compensatory 
movements. These movements were studied by him in many vertebrate 
and invertebrate forms; they were found to exist in insects as well as 
Crustacea. Using the crayfish, he confirms Clark’s results to some extent, 
but finds that on blinding the animals and removing the otocysts a con- 
siderable angle of compensation still persists. This, together with the 
fact that insects, which lack otocysts, show the characteristic movements, 
he uses as an argument against the otocyst being an organ of equilibra- 
tion. Lyon also finds that upon rotation about a vertical axis there isa 
