ANODONTA — ALIMENTARY SYSTEM. 
171 
always symmetrically developed, but when one otocyst only exists, 
as occasionally happens, it is found to uniformly contain two otoliths. 
The Olfactory sense is located more especially on the roof of the 
branchial siphon at the base of the gills near the external orifice, but 
is not exercised by distinctly differentiated or externally visible 
organs, but by elongated epithelial sensory cells overlaying a portion 
of the visceral ganglia, although innervated by a nerve from each of 
the cerebro-pleuro-visceral commissures, which are said, however, to 
really originate from the cerebral ganglia. The special function of 
this organ has been considered to be to examine the inflowing current 
of water which bathes the gills and brings nutriment to the mouth. 
The Gustatory sense is probably not at all or very little exercised, 
as no discrimination appears to be made in the selection of food 
particles fi-om the general substances brought by the current, except 
such supervision as is exercised by the labial palps. 
Cephalic Visual organs are not present in the adult Anodonta, 
but traces of such eyes are probably present during the early stages of 
development, but these, if present, afterwards disappear, as when 
covered by the shell they are useless and unnecessary ; the animals 
are, however, acutely sensible to light and shade, the exposed surface 
being able to discriminate and discern the distinction and respond to it. 
The Alimentary system has its anterior ojjening in the transversely 
oval mouth or oral aperture, which 
is placed beneath the anterior 
adductor muscle, and dor.sally to the 
origin of the foot and surrounded 
by an anterior or dorsal and a 
posterior or ventral lip, which are 
continuous with the lining mem- 
brane of the month and gradually 
expand on each side to form the 
somewhat triangular palps, which 
are transversely ribbed on their 
inner faces and beset with vibratile 
cilia, producing a current and con- 
veying food particles along the 
ciliated groove at their base, which leads towards and into the mouth, 
the animal being totally dependent for sustenance upon the minute 
organisms and food particles brought thereby. The mouth is 
Kig. 33S.— Transverse section througli the 
anterior region of Ajiodofiia cygnca^ to show 
the relations of the stomach, the liver and its 
ducts and the origin of the lyphlosole of the 
first tract of the alimentary canal (after 
Howes). 
y". foot, with the first tract of alimentary 
canal ; 1. liver, surrounding stomach and 
showing the bile ducts on the left side ; p. labial 
palps ; st. stomach, showing entrance of 
oesophagus, etc. ; t. t^'phlosole. 
