One osphradium or branchial olfactory organ is usually present on each side, on either side of the anus on the inner wall of the mantle, near the base of the last gill.
In Lepidopleuridae an osphradium occurs at the base of each gill.
Near the base of the ctenidium is a patch of sensory epithelium innervated from the branchial nerve, forming a sense-organ called the osphradium, whose function is to test the water entering the branchial cavity.
It consists of the anus in the middle, a renal organ and renal aperture on each side of this, and a ctenidium outside or anterior to the renal organ, an osphradium being situated at the base of the ctenidium.
All have an osphradium, except the Auriculidae, which are terrestrial, and it is situated outside the pallial cavity in those forms in which water is not admitted into the lung.