AUDITORY OR EQUILIBRATING ORGANS. 
‘237 
contecta is comparatively long sighted, perceiving objects at a distance 
of thirty centimetres from the eye, or if the mollusk be crawling in 
the gloom the sudden advent of a bright light at a greater distance 
will cause it to at once retire within its shell. Cyclo^toma elegans 
can also distinguish the hand of the observer when within eight 
inches or twenty centimetres, and abruptly contracts within and 
closes the shell on its nearer approach. 
The Auditory or Equilibrating organs were quite unknown in the 
mollusca prior to their discovery in the Cephalopoda by the celebrated 
anatomist, John Hunter. In 1838 Siebold detected the same organ 
in Anndonta, Unio, and other Pelecypods, and afterwards in many 
(lastropods, and its resemblance, in some species, to the auditory 
organs of certain tish-emhiyos strengthens the generalization that 
the fully developed organs of the lower animals may often represent 
those of the higher forms of life in an early stage of their development. 
The sense of hearing in mollusks cannot, however, be very acute, as 
the so-called auditory organs, when jiresent, are reduced to their 
simplest expression, two small closed sacs, termed otocysts, filled 
Fig. 464. — Section of the otocyst 
of Neritina Jfuviaiilis (L.), highly 
magnified (after Boll). 
aud.il. otocystic or auditory nerve ; 
ep. epithelium ; ot. otoconia ; vi. 
muscular layer with connective tisr.ue. 
'"fFiG. 465. — Otocyst of Anodonta cygnea 
(L.), showing the cellular walls, the en- 
closed otolith, and more especially the 
exterior distribution of ihe otocystic 
nerve, highly magnified (after Simroth). 
with a limpid fluid, and each holding in suspension one or many 
crystalline and sometimes coloured calcareous concretions, which vary 
in size, form and chemical constitution in the different species. The 
epithelial walls which secrete the endolymph or humour filling the 
sac, consist of ciliated and sensory cells, the latter with fine rod-like 
processes connected directly with the auditory nerves, whose fibres 
are distributed over the walls of the vesicle ; while the ciliated 
cells by their action create the incessant quivering and oscillation of 
the otoliths or otoconia, their concussion or contact with the longer 
