SEMICIRCULAR CANALS AND THE VESTIBULE. 413 



ently affected by the same position or movement. So also in pro- 

 gressive movements forward the weight of the otoliths may be im- 

 agined to exercise a stress of some sort upon the hairs. This theory 

 has been the subject of much investigation, numerous experiments 

 having been made chiefly upon fishes and invertebrates.* Accord- 

 ing to some observers destruction of these sacs or section of their 

 nerves is accompanied by a distinct interference with the fish's nor- 

 mal equilibrium: the animal swims at times upon its back or side 

 and apparently loses its normal means of judging correctly its posi- 

 tion. In many invertebrates there is present a sac, known as the 

 otocyst, containing hair cells and otoliths. Its structure resembles 

 that of the vestibular sacs of the mammalian ear, and it has been 

 assumed that it has a similar function. Experiments by numerous 

 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 palamon, 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.f 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 larvae deprived of their 

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



* Consult the following papers: Sewall, "Journal of Physiology," 4, 339, 

 1884; Lee, ibid., 15, 311, 1893, and "American Journal of Physiology," 1, 

 128, 1898; Lyon, "American Journal of Psychology," 3, 86, 1900. 



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

 1901, xxxvi., No. 7. 



