Published online by Cambridge University Press: 11 June 2012
The stomodæum appears as an ectodermal invagination. The oesophagus develops from an outgrowth (posterior) and the radular sac from a ventral evagination of the floor of the stomodæum. The basal epithelium of the radular sac secretes the radular membrane and the teeth, while the dorsal epithelium contributes towards the hardening of the teeth. The subradular cavity is formed from a depression of the floor and the salivary glands from two evaginations of the roof of the stomodæum. The buccal cartilage is mesodermal, while the jaw is ectodermal. The blastopore closes. Only the posterior part of the archenteron takes part in the formation of the gut, from which the crop, stomach and the digestive gland develop. The anterior chamber of the crop develops from a diverticulum of the archenteron. The anterior and posterior lobes of the digestive gland arise from the anterior part of the primitive stomach dorsally and on the left side respectively; the posterior chamber of the crop from the anteroventral region of the primitive stomach, and the stomach from the posterior right side of its floor. The hind gut appears by the arrangement of cells proliferated from that part of the archenteron where blastopore closes. It opens anterodorsally into the primitive stomach in a 2½ mm. embryo; the anus appears in a late embryo about 12 days old.