PKENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 237 



put the body in natural equilibrium, so in the case of the first three 

 lobster larvaj, the attitude maiutaiued is an adaptation for the greater 

 stability of the free-swimming animal, as yet without static organs. 

 But when, in the next stage, the otocyst becomes functional, such an 

 adaptation is no longer necessary, and the sudden change to the un- 

 stable swimming position of the fourth larval stage results (Fig. C). 

 This is the more natural attitude, and is advantageous to the animal 

 in that it allows of greater speed in swimming. 



5. The Function of the Otoliths. 



At the time when the otocyst was regarded as an auditory organ, the 

 otoliths were supposed to act simply as intensifiers of the sound vibra- 

 tions, but viewing the sac as a static organ, tlie role played by the 

 otoliths must assume a different aspect. The fact that they are want- 

 ing in the Brachyura, which nevertheless exhibit strong powers of 

 orientation, might be used as an argument against their playing any 

 important part in equilibration. But as tliey are present in the larval 

 crab, and as they disappear only when the otocyst becomes highly 

 differentiated, and when sensory hairs much more delicately constructed 

 than the otolitli bristles are developed, this argument loses most of its 

 weight. 



For determining the functions of the otoliths two methods may be 

 employed : (a) Observing the effect on equilibration and orientation 

 following the removal of the otoliths, or the prevention of the normal 

 pi'ocess of taking them in after ecdysis. (b) Substitution of iron dust 

 or iron filings for the otoliths, and the employment of an electro-magnet 

 to modify the action of gravity. If the otoliths are static in function, 

 the animals should orient themselves with reference to the resultant of 

 the attraction of the magnet, and the pull of gravity. 



The first of these methods was attempted by Kreidl ('94), but failed, 

 as he was unable completely to remove tlie otoliths. His results with 

 the second method of experimentation were definite and affirmative. 

 Lyon ('99) attempted to repeat and verify Kreidl's work, but his experi- 

 ments were incomplete and negative in their results. 



Otoliths, always normally present in macruran decapods, are lacking 

 for only a short time after ecdj'sis. So short indeed is this interval, 

 that it is extremely diflScult to find otocysts of newly moulted animals 

 which are without otoliths. Nor is it usually possible to prevent a 

 crustacean which has been observed to cast its test, from getting new 

 otoliths into the sac ; at least not for a sufficiently long period to allow 



