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.
In these there are neither branchia nor osphradium, and the pallial chamber which retains its large open ing serves as a lung.
It will be remembered that, according to Spengel, the osphradium of mollusca is definitely and intimately related to the gill-plume or ctenidium, being always placed near the base of that organ; further, Spengel has shown that the nerve-supply of this olfactory organ is always derived from the visceral loop. Accord ingly, the nerve-supply FIG.
Limpet, and that of the g ' nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphradium, which lies in immediate relation on the right and on the left side to the right and left gill-plumes (ctenidia) respectively.
There is a single well-developed, often pectinated osphradium.