PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 245 
22, Equilibration is made possible by three sets of organs, the oto-/ 
cysts, the eyes, and the tactile bristles. 
23. In free-swimming decapods the otocyst is by far the most im- 
portant of these static organs functionally, vision being secondary to it. 
Four facts go to prove this: 
(a) The removal of the otocysts causes a much greater loss of power 
of orientation, and a greater decrease in the compensatory movements 
of the eyestalks, than the loss of vision. 
(6) Decapods and Entomostraca normally without otocysts either 
swim in unstable equilibrium, or in a position identical to that which 
an inanimate object of the same form and weight would take under 
the influence of gravity. 
(c) Lobster larvee without functional otocysts are unstable in their 
swimming movements, but orient themselves with great accuracy at 
the stage when the sac becomes an active sense organ. 
(d) If iron filings are substituted for the otoliths, and an electro- 
magnet is employed to modify the effect of the pull and direction of 
gravity, shrimps orient themselves with reference to the direction of 
the resultant pull of the two forces, precisely as they do to the 
attraction of gravity alone. 
24. In lobster larvee of the third and fourth stages there is a direct 
correlation between the metamorphosis of the otocyst from a func- 
tionless to an active organ, and the changes in the swimming position 
of the animal. When the sac is inactive (third stage), the swimming 
position of the body and appendages is an adaptation which places the 
larve in comparatively stable equilibrium. As the otocyst becomes 
functional (fourth stage), this adaptation is no longer necessary, and 
a much less stable position is maintained, but one more favorable for 
rapid locomotion. 
25. The otoliths in Macrura and larval Brachyura are the means 
by which the pull of gravity is transmitted to the hairs of the otocyst. 
On their complete removal there is loss of equilibration and power 
of orientation ; if iron filings are substituted for them, shrimps may 
be made to respond to the attraction of an electromagnet. 
26. In adult Brachyura otoliths are normally lacking. The otolith 
hairs have become practically functionless, and the thread hairs are 
modified in such a way as to make them directly responsive to the 
attraction of gravity without the aid of the otoliths. 
VOL. XXXVI. — NO. 7 6 
