PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 229 
In the first case merely the mechanical action of gravity is called 
into play; in the second instance, besides the outside action of a 
physical agent, a subjective sense of direction and orientation is 
involved. 
In free-swimming decapods the body, moving or at rest, is in a 
position of unstable equilibrium. The dorsal side being always kept 
uppermost, the centre of gravity is high up, and a dead individual or 
an inanimate object of the same size, form, and disposition of weight 
would at once turn over. These animals must then by some means 
be rendered sensible to the direction of gravity, in order to be able 
to maintain a definite position of unstable equilibrium with reference 
to it. To determine what are the organs which perform the function 
of equilibration, the following means have been employed in the present 
investigation : 
(1) Removal, or prevention of the action of an organ, and observa- 
tion of the effects on the equilibration of swimming or walking decapods. 
(2) Observation of the effect of such removal on the gimbol-like 
movements of the eyestalks (compensation movements) when the 
animal is rotated about its different axes. 
(3) Observations on the orientation of animals normally without 
otocysts. 
(4) The effect of the development of the otocyst on the equilibration 
of the free-swimming larve. 
(5) The effect on equilibration of the addition of magnetic attraction 
acting on the otocyst at right angles to the pull of gravity. 
In these experiments blinding was accomplished by painting the 
eyestalks with a mixture of lampblack and shellac. The otocysts 
were removed under the lens of a dissecting microscope with the aid 
of a fine needle, bent in the form of a hook. Other parts, such as 
flagella of antennz and antennules, were simply cut off with a pair 
of fine scissors. Palemonetes vulgaris, being hardy, was the species 
chiefly employed, but experiments of a like nature were also carried 
on with Mysis, Crangon, and Gelasimus. A large number of trials 
were made with each species. When organs were cut off or destroyed, 
the animals so operated upon were kept under observation for from 15 
to 25 days, and the experiments were then repeated, in order to make 
sure that the effects observed directly after the operation were not due 
to abnormal conditions produced by nervous shock. 
VOL. XXXVI. — NO. 7 5 
