much larger than the other. The right kidney also has a sponge-like texture whereas the left kidney is essentially a small sac into which hang folds from the sac's walls. They do not have ctenidia, instead obtaining oxygen through a ring of gill lamellae that encircle the mantle just inside the shell edge and from the surface of the roof of the nuchal cavity which is exposed to air when the animal is no longer under water and which is covered in a network of blood vessels all of which eventually carry oxygenated blood and connect to the auricle through a series of veinlets on the animal's left side. Vestigial ctenidia have been adapted into osphradial patches (one on each side of the mantle cavity) with which the animal can "smell". Their low dome-shaped shell is able to withstand the forces of turbulent intertidal water. Inside, the head bears two tentacles, each with a tiny black "eye spot" at its base (limpets can sense light but cannot see images with these eyes). The heart lies within a pericardium and is composed of a single (morphologically left) auricle, a single ventricle, and bulbous aorta which sends blood to both the anterior and posterior aortae. It lies near the surface of shell on the left, and opposite it on the right are three tubules or "papillae" in a row: that of the left kidney, the anus, and that of the right kidney: all three exit near the same place on the right posterior side inside the mantle cavity. Between these papillae and the heart lies the neural "visceral twist", a nervous condition called streptoneury or chiastoneury, which characterizes many molluscs and all gastropods whose ancient ancestor had an anus located posterior to its head but which now have it positioned much closer because of a change in the arrangement of the shell. In the evolutionary course of the relocation of the anus, the various ganglia posterior to the pleural and pedal ganglia had to conduct a twist— this means, for example, that the osphradium on the animal's left side is innervated through the right side of its body and vice versa. The condition is called streptoneury, but the phenomenon is known as torsion. In the Patellogastropoda, the twist is located directly behind (i.e., posterior to) the pleural ganglia; in other closely related groups (e.g., Zeugobranchia, Neritopsina, and Ampullariidae) the twist stretches backwards