Mechanisms leading to the separation of small particles (including microorganisms), water-soluble substances and water from large dry matter particles occur in the large intestine in rabbits, poultry and some small rodents such as the Scandinavian lemming, the chinchilla and the guinea pig. In rabbits water secreted in the proximal colon and absorbed in the caecum together with antiperistaltic movements of the wall of the haustra wash out and transport fine matter back to the caecum, whereas coarse matter continues in the anal direction and is delivered in hard faecal pellets. In some rodents the anatomical arrangements allow a selective delay of microorganisms in the caecum and the most proximal part of the colon. In poultry selective filling of the caeca together with retrograde transport of urine from the cloaca to the caeca allow only fluid and fine matter to pass into the caeca, whereas coarse matter is delivered in the ordinary faeces. Separation and retrograde transport result in both a high average rate of transport of food and food residues through the digestive tract and a relatively high degree of digestibility, allowing these animals to live on food rich in fiber and low in easily digested nutrients.