414 THE SPECIAL SENSES. 



observers have indicated that when the otoliths are removed the 

 animal shows disturbances in equilibrium, particularly in the matter 

 of the compensatory movements exhibited during rotation. Others, 

 however, deny these facts and state that invertebrates without oto- 

 cysts make compensatory movements when rotated and that in 

 those with otocysts compensatory movements and maintenance of 

 normal equilibrium persist after destruction of the sacs. A very 

 ingenious experiment reported by Kreidl seems to show that the oto- 

 liths may affect the hairs by their weight. When the palsemon, a 

 crustacean, molts it casts off the inner lining of the otocyst, together 

 with the otoliths. The otocysts in these animals lie at the base of 

 the antennules and open freely to the exterior. After molting the 

 animal by means of its claws places fine grains of sand in the otocyst 

 to act as otoliths. Taking advantage of this peculiarity, Kreidl 

 placed the animal, after molting, upon finely powdered iron, with 

 the result that some of the iron granules were deposited in the oto- 

 cyst in place of the usual grains of sand. When now a magnet was 

 brought near to the animal reactions were obtained which showed 

 that the pressure of the iron upon the hairs influenced its position. 

 The position taken by the animal under these conditions was such 

 as would be expected as a resultant of the forces of magnetism and 

 gravity, and the experiment, therefore, justifies the hypothesis that 

 under normal conditions gravity affects the otoliths and through 

 them the muscular co-ordination of the animal. These experiments 

 have been confirmed by Prentiss.* This author has shown, 

 moreover, that if larval lobsters (4th stage) are prevented from 

 obtaining otoliths after moulting by placing them in filtered sea- 

 water, their movements, like those of larvse deprived of their 

 otocysts, show a distinct instability and lack of normal orientation. 

 Some interesting results reported by Streeter,* from experiments 

 made upon tadpoles, demonstrate in a striking way the impor- 

 tance of the labyrinth to the power or property of equilibration. 

 Up to the sixth day after fertilization the tadpole shows no evi- 

 dence of a power of equilibrium, and is not capable of swimming 

 freely in the water. When the auditory vesicles appear, after the 

 sixth day, the animal exhibits evidence of a sense of equilibrium 

 and swims freely in the water out of contact with solid substances. 

 If at or before the beginning of this stage one of the auditory 

 vesicles is removed, the tadpole shows evident, although tem- 

 porary, signs of a defective power of equilibration. If both 

 vesicles are removed the animal seems to lose permanently its 

 power of swimming, and when at rest lies in any position, as though 

 it were lacking in a sense of equilibrium. 



* Prentiss, " Bulletin of Museum of Comparative Zoology," Harvard, 

 1901, xxxvi, No. 7. 



t Streeter, " Journal of Exp. Zoology," 3, 543, 1906; also vols. 4 and 16. 



