236 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



of the thoracic appendages have been lost, and the larva now swim 

 swiftly by means of the abdominal swinimerets. 



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 shai'p contrast to the preceding stages. 

 AH 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 eai'lier 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 larvae. 



If larvae 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 

 bv lobster larvte 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) foinid 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, 



