PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 237 
put the body in natural equilibrium, so in the case of the first three 
lobster larve, the attitude maintained 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 Funetion 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, the 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 they 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 otolith 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 
process of taking them in after ecdysis. (6) 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 the 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 ecdysis. So short indeed is this interval, 
that it is extremely difficult 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 
