The main sensory organ of prosobranch molluscs is a structure with many folds called osphradium, located in the gill cavity. The current or water passes through the osphradium allowing the snail determine some aspects of the chemical composition of the water that surrounds it (the smell). Only the most primitive opisthobranchs still have the osphradium because the immense majority have evolved and have developed new sensory structures such as sensory mounds or oral tentacles, around the mouth.
The most obvious sensory organs in the opisthobranchs are the rinophores, which are horn-shaped structures they have on the head. Little is known about the function of rinophores, although it seems they are very important organs. The tissue that covers them is not much different from other parts of the body but for the absence of secretory glands. Although there is no direct evidence, most authors agree that the role of rinophores is to find food and other individuals of their same species to reproduce.
In Anaspidea, Notaspidea and most Sacoglossa, the rinophores are rolled and look like rabbit ears. The inside face is covered with ciliated cells to promote water movement. Epithelium, the tissue of the outer side, is covered with nerve terminations in the form of receivers.
Detail of anaspidean (Aplysia punctata) rhinophores
with the shape of rabbit ears
The Cephalaspidea lack rinophores, so this function is assumed by an external folded structure, the Hancock’s organ, or a kind of sensory funnel.
The Nudibranchia have more complicated rinophores. The clavus or distal end is usually composed of a series of folds like gills, although some are smooth and others have greatly elaborated designs. In some species the surface is covered in ciliated cells to cause water movement. Rinophores’ nerves are highly branched and connect directly to the cerebral ganglion. The major group of doridacean nudibranchs (Cryptobranchia) the rinophores can retract inside the body, pressumably for protection.