PEENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 229 



lu 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 weio-ht 

 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) Kemoval, or prevention of the action of an organ, and observa- 

 tion of the eiFects 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 larvae. 



(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 antennae and antennules, were simply cut off with a pair 

 of fine scissors. Palaemonetes vulgaris, being hardy, was the species 

 chiefly employed, but experiments of a like nature were also earned 

 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 2.5 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 



