412 THE SPECIAL SENSES. 



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 otoHths are removed the animal 

 shows disturbances in equilibrium, particularly in the matter of 

 the compensatory movements exhibited during rotation. A very 

 ingenious experiment reported by Kreidl gives direct evidence that 

 the otoliths may affect the hairs mechanically. When the palse- 

 mon, a crustacean, molts it casts off the inner lining of the otocyst, 

 together with the otohths. The otocysts in these animals lie at the 

 base of the antennules and open freely to the exterior. After the act 

 of molting the animal by means of its claws places fine grains of sand 

 in the otocyst to act as otohths. Taking advantage of this peculiar- 

 ity, Kreidl placed the animal, after molting, upon finely powdered 

 iron, with the result that some of the iron granules were deposited 

 in the otocyst 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 condi- 

 tions 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 (fourth 

 stage) are prevented from obtaining otohths after molting 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,t from experiments made upon tadpoles, demonstrate in 

 a striking way the importance of the labyrinth to the power or 

 property of equilibration. Up to the sixth day after fertilization 

 the tadpole shows no evidence 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 con- 

 tact with solid substances. If at or before the beginning of this 

 stage one of the auditory vesicles is removed, the tadpole shows 

 evident, although temporary, signs of a defective power of equili- 

 bration. If both vesicles are removed the animal seems to lose 

 permanently its power of swimming, and when at rest hes 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. 



