2-40 bulletin: museum of compakative zoology. 



auimals during the experiments. I believe there is only one explanation 

 for this turning of the body away from the attracting force, and that is 

 a very simple one. Under normal conditions the body of the shrimp 

 is oriented with reference to gravity, and its dorso-veutral axis ap- 

 proximately corresponds to the direction of this force. If the shrimp 

 rotates around its chief axis either to right or left, say 90°, the 'direc- 

 tion of the pull of gravity on the otoliths is at once changed, and through 

 the medium of the latter other sensory hairs of the sac are stimulated. 

 As a result, the shrimp turns back in a direction opposite to that in 

 which it was rotated, until it is again in a normal relation to the 

 direction of gravity. The employment of the magnet has no other 

 effect than merely to change the direction of the orienting force. This 

 is now no longer that of gravity alone, but the resultant of the two com- 

 ponent forces, gravity and the pull of the magnet. The animal now 

 maintains its swimming position in reference to this new line of attrac- 

 tion, its dorso-ventral axis coincident with that line, and as a result the 

 dorsal side is turned away from the magnet. To put it in another way, 

 when the magnet is held close to the right side of the otocyst, the 

 animal is stimulated precisely as it would be if rotated to tlie right 

 45°, and it responds as it would normally in righting itself, i. e., by 

 turning its body in the opposite direction through an angle sufficient 

 to make its dorso-ventral axis coincide with the direction of the attrac- 

 tive force ; in this case through an angle of 45°. 



This single series of observations completely confirm, as far as they 

 go, the very important conclusions of Kreidl. The otoliths are found 

 to play an important part in the functional activities of tlie otocyst, and 

 the latter is conclusively proved to be a static organ, acted upon by the 

 force of gravity; this force makes itself felt chiefly through the medium 

 of the otoliths, and if they are absent, as described in a preceding set of 

 experiments on lobster larvae, the function of the otocyst in Macrux-a 

 is seriously impaired. 



6. The Function of the Hairs of tlie Otocyst. 



The function of the otocyst hairs of macruran decapods which are in 

 contact with otoliths has been already briefly discussed in.the first part 

 of this paper. The stimulus imparted to the hair shaft through the 

 medium of the otoliths makes itself most strongly felt at the labile base 

 of the hair, owing to the rigidity of the shaft and the delicacy of the 

 attaching membrane. At this point, too, the nerve fibre invariably ends, 

 and the stimulus is thus transmitted to it, and at once carried to the 



