Al’DITORY ORGANS — OTOLITHS AND OTOCONIA. 
‘239 
The otocysts originate in the embryo as simple paired invagina- 
tions of the outer epithelium of the foot, the canal of invagination 
or Kblliker’s canal, as it is termed in the Cephalopods, persisting only 
in the adult Nucain and other primitive Pelecypods, and opening 
externally at the anterior end of the foot, the otoconia being- 
composed of grains of sand or other extraneous particles. 
The calcareous concretions or “hearing stones” contained within 
the sacs are, like the calcareous basis of the external shell, usually 
composed of that form of carbonate of lime termed Aragonite, and 
dii’ectly originate from the cells of the sac. 
They may be classified for the purposes of study as (Otoliths and 
Otoconia, according to the number and character of the concretions 
contained in the cysts. 
The Otoliths (o?s, ear; Xi0os, stone) are comparatively large, often 
solid, and more or less spherical con- 
cretions, usually distinguishalde from 
the calcareous spherical granules found 
in other parts of the body or mantle 
by their concentric structure and radial 
striation ; they always exist singly within the cysts, and do not increase 
in number with the age of the animal. 
Fig. 469.— Otolith from the otocyst 
of SphcFriuin cornezan (L.), higlily 
magnified (after Simrotb). 
The Otoconia (ocs, ear ; /coi-ta, dust) are smaller, more numerous, 
and more variable in shape than the otoliths, and may be of a simple 
or compound form; the simple form embraces all otoconia with a 
regular and simple outline, as those characterizing llijalbua and other 
genera ; the compound forms are distinguished by distinct and usually 
symmetrical divi.sions or segments, each .showing a minute central 
area, the demarcation into two or four divisions predominating. 
Figs. 170. 171. 172. 173. 171. 175. 176. 177. 178. 17!). 180. 181. 
Simple and compound otoconia from the otocysts of various species of British Euthyneurous 
mollusca to show some of the differences in form, highly magnified (after Schmidt). 
Figs. 170—172 represent the prevailing forms in the Stylommatophora generallj-. Fig. 173 more’ 
particularly belongs to the Basommatophora. Fig. 171 is more especially found in Hyalinia.‘ 
Figs. 175, 176 are from Helix pomatia, but these peculiar forms are only occasionally found 
and are also not confined to that species. Figs. 177- 180 are similarly from Helix hortensis, 
Fig. 180 being an end view of Fig. 179, and Fig. 181 is from Limax cinereo-niger. 
while the partition into three, five or six segments is less frequent. 
The number in each cyst also increases with the age of the animal, 
in an embryo of Limnwa atagnalk Siebold counted only fifteen of 
