PKENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 245 



22. Equilibratiou 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. 



(b) Decapods and Entomostraca normally without otocysts either 

 swim in unstable equilibi'ium, 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 larvae 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 larvae 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 

 larvae 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 ]\racrura 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 



