236 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
of the thoracic appendages have been lost, and the larve now swim 
swiftly by means of the abdominal swimmerets. 
Although, from the position in which the body and appendages are 
held, the larva is in unstable equilibrium, it now orients itself very 
definitely during locomotion, in sharp contrast to the preceding stages. 
All signs of rolling from side to side, or pitching forwards, are com- 
pletely lost. The larva swim straight ahead with the body held 
usually in a horizontal plane and dorsal side up. The same position is 
also invariably maintained when the animals come to rest. 
Thus this sudden change as to form and swimming position in the 
fourth larva, unfavorable though it is for equilibration, is yet accom- 
panied by more delicate powers of orientation, and greater stability in 
swimming than are met with in the three earlier stages, where the 
centre of gravity of the animal is lower. Bearing in mind the fact that 
the otocyst first becomes functional in the fourth larval stage, we can 
only conclude that an intimate connection exists between its appearance 
as an active organ, and the delicate static sense which is suddenly 
exhibited by the larvee. 
If larve of the first, second, and third stages are blinded, their 
powers of orientation are almost entirely lost, but the same experiment 
has little or no effect upon the equilibration of the fourth larva. The 
first three stages thus depend mainly on vision for their imperfect ori- 
entation ; in the next stage this function has been largely transferred 
to the otocyst. 
A similar correlation between the development of the otocyst and the 
appearance of a static sense is found in the metamorphosis of the crab. 
The pelagic unstable Zoea larva is without otocysts, while the Megalops 
larva, which exhibits perfect powers of equilibration, possesses these 
organs well developed, and even containing otoliths, which are absent 
in the sac of the adult. 
The correlation which evidently exists between the formation of the 
functional otocyst and the sudden increase in static powers exhibited 
by lobster larvee is particularly well shown in the marked alteration in 
the swimming position maintained by the fourth larva, as compared 
with that of the three earlier stages. Previous to the fourth stage, the 
lack of a delicate static organ is compensated for by the maintenance 
of an attitude in swimming which increases the stability of the moving 
body. Just as Bethe (95) found that Mysis, deprived of its otocysts, 
would after an interval of some days recover its power of orientation by 
curving the abdomen upward and thus. by lowering the centre of gravity, 
