3. _The Oesophagus._--That part of the alimentary canal which lies between the pharynx and the stomach (in Pelecypoda between the mouth and stomach) is known as the oesophagus. Its exact limits are not easy to define, since in many cases the tube widens so gradually, while the muscular structure of its walls changes so slowly that it is difficult to say where oesophagus ends and stomach begins. As a rule, the oesophagus is fairly simple in structure, and consists of a straight and narrow tube. In the Pulmonata and Opisthobranchiata it often widens out into a ‘crop,’ which appears to serve the purpose of retaining a quantity of masticated food before it passes on to the stomach. In _Octopus_ and _Patella_ the crop takes the form of a lobular coecum. In the carnivorous Mollusca the oesophagus becomes complicated by the existence of a varying number of glands, by the action of which digestion appears to begin in some cases before the food reaches the stomach proper. 4. _The Stomach._--At the posterior end of the oesophagus lies the muscular pouch known as the stomach, in which the digestion of the food is principally performed. This organ may be, as in _Limax_, no more than a dilatation of the alimentary canal, or it may, as is usually the case, take the form of a well-marked bag or pocket. The two orifices of the stomach are not always situated at opposite ends; when the stomach itself is a simple enlargement of the wall of one side of the alimentary canal, the cardiac or entering orifice often becomes approximated to the orifice of exit (pyloric orifice). The walls of the stomach itself are usually thickened and strengthened by constrictor muscles. In some Nudibranchs (_Scyllaea_, _Bornella_) they are lined on the inside with chitinous teeth. In _Cyclostoma_, and some _Bithynia_, _Strombus_, and _Trochus_ there is a free chitinous stylet within the stomach.[328] In _Melongena_ (Fig. 146) the posterior end of the oesophagus is provided with a number of hard plate-like ridges, while the stomach is lined with a double row of cuticular knobs, which are movable on their bases of attachment, and serve the purpose of triturating food.[329] _Aplysia_ has several hard plates, set with knobs and spines, and similar organs occur in the Pteropoda. But the most formidable organ for the crushing of food is possessed by the Bullidae, and particularly by _Scaphander_ (Fig. 145). Here there is a strong gizzard, consisting of several plates connected by powerful cartilages, which crush the shells, which are swallowed whole.