MOLLUSCA 195 
The nerve ganglia are usually rendered conspicuous by 
their bright orange colour. The cerebral ganglia, which lie 
one on each side of the mouth, probably represent the cerebral 
and pleural ganglia of other molluscs; they are united 
both with the pedal ganglia in the foot and with the olfactory 
(parieto-splanchnic) situated on the ventral face of the posterior 
adductor muscle. A pair of auditory vesicles, lined with 
ciliated cells and containing a single otolith, are usually 
present close to the pedal ganglia, and are innervated by a nerve 
from the cerebro-pedal commissure, which probably comes from 
the cerebral ganglia. Tactile papillae or tentacles are common 
round the edge of the mantle. In some cases the tentacles 
have been modified and form eyes, which attain a great degree 
of complexity. In Pecten, Spondylus, etc., these eyes have a 
remarkable resemblance to the vertebrate type of eye, inasmuch 
as the optic nerve passes in front of the retina, and the retinal 
elements are thus turned away from the light. The epithelium 
in the neighbourhood of the olfactory ganglion is modified to 
form an organ of smell, by means of which the quality of the 
water flowing in through the ventral siphon may be tested. 
The Lamellibranchs with few exceptions are dioecious. 
The generative organs are branched glands usually situated in 
the foot, though in Mytilus they occur in the mantle. The 
generative cells are formed in the caecal processes of the gland, 
and they leave the body by a right and left simple duct which 
is continuous with the walls of the gland, and in some cases 
opens into the duct of the kidney (Spondylus, Lima, and 
Pecten). 
Division II]. GLOSSOPHORA. 
CHARACTERISTICS.—WMollusca with a prostomium more or less 
developed and a buccal cavity armed with a rasping tongue, 
the radula, which together with its accessory parts constitutes 
the odontophore. 
The Glossophora comprise three classes : 
(i.) Gasteropoda. 
(ii.) Scaphopoda. 
(iii.) Cephalopoda. 
