32 HENSEN, ON THE AUDITORY ORGAN 
former case, which is that of the organ in the erab, lobster, 
prawns, and shrimps, &c., the otolites are of an irregular 
form, and resemble grains of sand. Some authors, conse- 
quently, have been induced to regard these particles as foreign 
bodies, whilst others look upon them as integral parts of the 
animal. M. Hensen has succeeded in settling this question 
definitely, by examining the Crustaceans in question at the 
time of moulting. At this period the old otolites are thrown 
off, together with the chitinous tunic, and replaced by others. 
Shrimps examined immediately after they have moulted have 
no otolites, but a few hours afterwards minute, irregular, 
strongly refractive particles, either siliceous or calcareous, may 
be observed within the auditory sac. On such an occasion M. 
Hensen has seen a shrimp scraping the bottom of the glass 
with its pincers, which it afterwards introduced into the 
auditory sac, but he was unable to witness the actual intro- 
duction of the minute lapilli. In order to remove all doubt 
about the matter, he placed some shrimps in a vessel contain- 
ing filtered sea-water, and at the same time covered the bot- 
tom with crystals of uric acid. Ina short time one of the 
shrimps moulted, and the auditory sacs of the discarded shell 
were found to contain the ordinary grains of sand, but no 
uric acid. Three hours later M. Hensen examined the 
animal, and found that the sacs contained numerous crystals 
of uric acid, without a single grain of sand. It is manifest, 
therefore, that in Crustacea with an open auditory sac the 
otolites are derived from without, as had been previously 
stated by M. Favre. 
In the Crustacea with a closed sac the otolites have no re- 
semblance whatever to grains of sand. They constantly pre- 
sent a peculiar structure, and their surface is never irregular. 
Nevertheless, even in this case, their duration is only tem- 
porary. In fact, the Crustacea in question periodically 
throw off the auditory sac, together with the otolites, and the 
whole apparatus is formed anew. 
The inorganic constituent of these otolites is a calcareous 
salt, but it is neither a carbonate, nor a phosphate, nor a 
silicate, nor a sulphate. Its reactions seem to indicate that 
it consists of fluoride of calcium. 
The cavity of the auditory organ is always lined with a 
delicate chitinous membrane, which should be regarded as 
an inversion of the external chitinous coat. This holds good 
even in the case of the closed sacs, which have no communi- 
cation whatever with the exterior; and in these it is possible 
to discern a line or sort of cicatrix, the vestige, as it were, of 
the point at which the invagination of the sac took place. 
