The essential feature of the asymmetry of Gastropoda is the atrophy or disappearance of the primitively left half of the circumanal complex (the right half in sinistral forms), including the gill, the auricle, the osphradium, the hypobranchial gland and the kidney.
Two osphradia present but no hypobranchial glands nor operculum.
RHIPIDOGL0ssA.-Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single.
In addition to the characters given above, it may be noted that the mantle is provided with a hypobranchial gland on the outer side of each gill, the auricles are muscular, the kidneys are glandular through their whole length, the sexes are separate.
The ventral branchial vessel lies below the hypobranchial groove or endostyle, and is the representative of a heart.