Pharynx and oesophagus are concealed in the head.
A buccal cavity, a pharynx, an oesophagus and an intestine are always distinguishable.
In medicine, nitric acid is used externally in a pure state as a caustic to destroy chancres, warts and phagadenic ulcers; and diluted preparations are employed in the treatment of dyspepsia, &c. Poisoning by strong nitric acid produces a widespread gastroenteritis, burning pain in the oesophagus and abdomen and bloody diarrhoea.
A so-called heart lies on the dorsal surface of the oesophagus; it is closed behind, but in front it opens into a circumoesophageal ring, which gives off vessels into the lophophore and tentacles.
The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally.