In the case of a species of Fusus the hypoblast, when enclosed by the epiblast, is composed of four cells only. The blastopore remains permanently open at the oral region, and around it the oesophagus grows in a wall-like fashion. The protoplasmic portions of the four hypoblast cells are turned towards the oesophageal opening, and from them are budded off small cells which are continuous at the blastopore with the epiblast of the oesophagus. These cells give rise posteriorly to the intestine and anteriorly to the sack, which becomes the stomach and liver. This sack always remains open towards the four primitive yolk cells. The cells of the posterior part of it become larger and larger and form the hepatic sack, which fills up the left and posterior part of the visceral sack, pushing the yolk cells to the right. The cells lining the hepatic sack become pyramidal in shape, and each of them is filled with a peculiar mass of albuminous material. The cells adjoining the opening of the oesophagus remain small, become ciliated, and form the stomach. They are not sharply separated off from the cells of the hepatic sack. The yolk cells remain distinct on the right side of the body during larval life, and their food material is gradually absorbed for the nutrition of the embryo. A modification of the above mode of development, where the food material is still more bulky and the blastopore closed, is found in Nassa, and has already been described (_vide_ p. 233). _The stomodæum._ The stomodæum in most cases is formed as a simple epiblastic invagination which meets and opens into the mesenteron. When the blastopore remains permanently open at the oral region the stomodæum is formed as an epiblastic wall round its opening. In all cases the stomodæum gives rise to the mouth and oesophagus. At a subsequent period there are developed in the oral region of the stomodæum the radula in a special ventral pit, and the salivary glands--the latter as simple outgrowths. The oesophagus is usually ciliated. _The proctodæum._ Except where the blastopore remains as the permanent anus (Paludina) the proctodæum is always formed subsequently to the mouth. Its formation is usually preluded by the appearance of two projecting epiblast cells, but it is always developed as a very shallow epiblastic invagination, which does not give rise to any part of the true intestine. In the Cephalopods the alimentary tract is formed, as in other cephalophorous Mollusca, of three sections. (1) A stomodæum, formed by an epiblastic invagination, which gives rise to the mouth, oesophagus and salivary glands. (2) A proctodæum, which is an extremely small epiblastic invagination. (3) A mesenteron, lined by true hypoblast, which forms the main section of the alimentary tract, viz. the stomach, intestine, the liver, and ink sack[116]. [116] The following description applies specially to Loligo.