238 bulletin: museum of comparative zoology. 



the animal otherwise to regain its normal condition. Even if placed at 

 once in filtered water, some otoliths soon make their appearance, probably 

 originating from the excreta of the animals themselves. 



In lobsters the larviB regain their normal condition within a much 

 shorter interval after ecdysis than do adult individuals ; their digestive 

 tract is also much less likely to contain material suitable for the forma- 

 tion of otoliths. Therefore, after tiying in vain to completely remove 

 the otoliths from the sacs of Crangon and Pahemonetes, my attention 

 was directed to lobster larvee as much more favorable material than 

 the adult shrimps. As they moult at intervals of a few days, it is also 

 much easier to obtain them directly after ecdysis or in the very act 

 itself. So obtained, and placed at once into filtered sea water, larvae of 

 the fourth stage may be kept without otoliths for from twenty-four to 

 forty-eight hours, and a favorable opportunity is thus given for observing 

 the effect produced by the lack of otoliths on the equilibrium of the 

 animals. 



Observations were made on eighteen larvse of the fourth and fifth stages, 

 all of them being kept free from otoliths for at least twelve hours. Within 

 two hours after moulting most of them swam about actively, and ate 

 greedily when fed with bits of crab's liver. In swimming, however, they 

 show distinctly the phenomena manifested by shrimps which have been 

 deprived of their otocysts. There is both " rolling " from side to side, 

 and " pitching " forward and backward ; often they swim with the 

 ventral side uppermost. Much moi*e easily overturned than normal 

 larvse, they do not right themselves at once, but if turned upon the 

 back, will continue to swim in that abnormal position. If blinded, 

 the loss of equilibrium is still more marked. All these conditions 

 are in strong contrast to the actions of the normal free-swimming larvse 

 of these stages, which conduct themselves in the characteristic manner 

 already described for Palsemonetes. 



The observations having been made and recorded, the animals were 

 killed, and the otocysts dissected out and examined under the micro- 

 scope. Scarcely a particle of inorganic matter was found in the sacs 

 of sixteen larvse. In two individuals a few small grains of sand were 

 found in one otocyst, but the other was entirely destitute of otoliths. 



From the number of cases observed it seems safe to conclude that the 

 otoliths do play an important part in equilibration, and that it is the 

 pull of gravity upon them which stimulates the sensory hairs of the sac. 

 If the loss of the power of accurate orientation were to be attributed to 

 the abnormal conditions resultant upon ecdysis, it might be said in 



