United with these ganglia on the outer sides are the osphradial ganglia, above which the epithelium is modified to form a pair of sense-organs, corresponding to the osphradia of other Molluscs.
A pair of pedal otocysts, and a pair of osphradia at the base of the gills, appear to be always present.
The special sense-organs are a pair of eyes on the head, a pair of otocysts or statocysts, and a pair of osphradia which have already been mentioned.
The sense-organs of Anodonta other than the osphradia consist of a pair of otocysts attached to the pedal ganglia (fig.
O, o, Right and left olfactory ce.pl, The cerebro-pleural conganglia and osphradia re nective.
RHIPIDOGL0ssA.-Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single.
Two osphradia present but no hypobranchial glands nor operculum.