1. The description of the gross morphology of the gut of Mitopus morio agrees with Kastner's (1933) description for Opilio parietinus De Geer except that 4 pairs of lateral openings lead from the mid-gut into the diverticula and not 3 as previously recorded.
2. The two main types of diverticular cell in M. morio are digestive and secretory. There is thus agreement with the structures present in spiders, scorpions, and other harvest-spiders.
3. The mid- and hind-gut epithelia both possess a single cell-type. That of the midgut is absorptive and digestive, whereas that of the hind-gut secretes a membrane round the faeces to form a pellet.
4. During protein metabolism some extracellular digestion occurs in the lumina of the diverticula and mid-gut. The necessary enzyme or enzymes are produced by the diverticular secretory cells and are replenished within 24 h.
5. Extracellular digestion is followed by partial or complete intracellular digestion in the diverticular digestive cells, which then cut off either digestive cell apices, containing partially digested protein globules, or digestive cell faeces which contain excretory material.
6. Further digestion of the protein globules of the digestive cell apices occurs in the mid-gut cells, which then produce mid-gut cell faeces that are voided with the digestive cell faeces and indigestible food remnants.