PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 241 
brain. In the case of adult Brachyura, however, there are no otoliths 
in contact with the hairs of the otocyst, consequently the effect of 
gravity, if not entirely null, must be at least greatly lessened, unless 
indeed the hairs are so differentiated as to be themselves stimulated 
by it. 
Bethe (’97), acting on the idea that in tilting the animal the differ- 
ence in the pressure of the water might affect the hairs of the otocysts, 
placed crabs under very high pressures where the slight difference brought 
about by tilting would be practically eliminated. But he found that all 
the phenomena of equilibration still persisted. 
It is probable that in the otocyst of Carcinus the thread hairs are the 
most important sensory organs of the sac. The hook hairs, originally 
in the larva attached to otoliths, later, with the loss of the sand granules, 
lose much of their functional activity ; the third group of hairs can- 
not be of great importance, as I could not demonstrate satisfactorily 
their nerve connections, and their structure alone is such as to preclude 
their being affected by very delicate stimuli. The thread hairs, how- 
ever, in both structure and position are fitted for the fulfillment of such 
a function as has been ascribed to them. The shaft is long, attenuate, 
only slightly fringed at the tip, and attached at the base by a very thin 
membrane, which allows free movement to the rigid shaft about this 
region as upon a joint. I have observed in studying freshly dissected 
otocysts that a slight tilting of the watch glass in which they were con- 
tained caused these hairs to sway extensively. 
From Clark’s experiments and my own, it was apparent that upon 
rotation in a horizontal plane, there was little or no compensatory 
movement of the eyestalks, and that when there is such a reaction, the 
angle of compensation is not maintained, but the eyes return at once to 
their original positions. Also, on rotation about the animal’s lateral 
axis, the angle of compensation is not as great, when the rotation is 
rapid and jerky, as when performed slowly and smoothly. These two 
facts preclude the possibility of the hairs being affected by movements of 
the fluid surrounding them, at least to any great extent. For if they 
were so affected, the angle of compensation should be the same, in what- 
ever plane the animals are rotated, and the position of the eyestalks 
should be in every case maintained by compensation movements. 
There still remain two ways in which the hairs may be so affected as 
to bring about nervous stimulus. Either they may be lighter than the 
surrounding fluid, and consequently tend always to float erect, no matter 
what position the otocyst may take relative to them; or they may be 
