184 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Aside from its general interest, this peculiar method of forming the 

 new hair is very important, as throwing light on the peripheral endings 

 of the nerve fibres in the sensory hairs. By it certain conditions may 

 be explained. At each moult the nerve fibres lose their connection 

 with the old hairs, and come into relation with new ones. How these 

 changes are brought about can best be described in connection with the 

 innervation of the otocyst. 



e. The Otoliths are borne in a rather compact mass upon the inter- 

 laced tips of the sensoi-y hairs (Plate 1, Fig. 3, otHth). They consist of 

 irregular grains of sand mingled with other fine mineral particles and 

 organic detritus. The largest measure from 8 to 12 ^u in longest dimen- 

 sion. That the greater part of them are siliceous is shown by their 

 insolubility in strong sulphuric acid, and by the fact that they scratch 

 glass when crushed upon it. They are renewed after each moult, 

 for the freshly formed sac is at first without them. New otoliths are 

 pushed in by means of the chelae through the aperture of the sac 

 while its walls are yet so soft and flexible as to admit quite large grains 

 of sand. By watching animals soon after moulting it can be observed 

 that they stir up the sand at the bottom of the aquarium in which they 

 are confined ; as soon as some particles have come to rest upon the 

 dorsal side of the antennule, one or both chelae are raised, and by their 

 tips the grains of sand are pushed back under the protecting lid of the 

 opening into the otocyst. Otocysts from which most of the sand parti- 

 cles had been carefully removed by forcing a jet of water into the sac 

 were found after a lapse of two days to contain otoliths derived from 

 iron filings which had been strewn on the bottom of the aquai'ium. The 

 otoliths are often entangled in the feathery plumes of the auditory hairs, 

 and are in this case attached to them by an organic substance, which is 

 probably secreted by unicellular glands situated beneath the floor of the 

 sac. No multicellular glands, such as are found in the lobster and cray- 

 fish, could be detected beneath the otocyst of Paltemonetes. Very 

 minute canals, which are probably the ducts of gland cells, were found 

 running through the chitin wall and some distance into the tissues 

 beneath ; they were very clearly brought out, and their tubular condi- 

 tion proved beyond a doubt, in silver preparations, and in tliose made 

 with lead formate ; but unfortunately their connection with gland cells 

 could not be demonstrated. The functions of the otolith and the 

 part it plays in audition, or equilibration, will be discussed in the 

 experimental portion of this paper. 



