Mesenteron

The midgut begins just past the stomodeal valve.   Near its anterior end, finger-like projections (usually from 2 to 10) diverge from the walls of the midgut.   These structures, the gastric caecae, provide extra surface area for secretion of enzymes or absorption of water (and other substances) from the alimentary canal.   The rest of the midgut is called the ventriculus -- it is the primary site for enzymatic digestion of food and absorption of nutrients.   Digestive cells lining the walls of the ventriculus have microscopic projections (microvilli) that increase surface area for nutrient absorption.

The midgut is derived from embryonic endoderm so it is not protected by an intima.   Instead, the midgut is lined with a semipermeable membrane secreted by a cluster of cells (the cardial epithelium) that lie just behind the stomodeal valve.   This peritrophic membrane consists of chitin fibrils embedded in a protein-carbohydrate matrix.   It protects the delicate digestive cells without inhibiting absorption of nutrient molecules.

The posterior end of the midgut is marked by another sphincter muscle, the pyloric valve.   It regulates the flow of material from the mesenteron to the proctodeum.