The third or outermost chamber, the proctodaeum, is closed externally by the sphincter ani; the orifice is quite circular.
The penis, and its much reduced vestige of the female, is developed from the ventral wall of the proctodaeum.
In some parasitic Hymenoptera there is only a single the proctodaeum respectively.
They have appeared independently in connexion with a change in the excretion of nitrogenous waste in Arachnids, Crustacea, and the other classes of Arthropoda when aerial, as opposed to aquatic, respiration has been established - and they have been formed in some cases from the mesenteron, in other cases from the proctodaeum.
After the formation of the gastrula by epibole the larva becomes enclosed by an ectodermic test covering the whole of the original surface of the body, including the shell-gland, and leaving only a small opening at the posterior end in which the stomodaeum and proctodaeum are formed.