Summary1Digestion in the primitive animals must have been intracellular, as it remains in the Protozoa and in the Porifera. It has persisted, to a greater or less extent, in a number of Metazoa. These may be divided into two groups: (1) those which are primitive in structure,e.g.Coelenterata, Ctenophora, most Turbellaria, andLimulus; and (2) those which are more highly evolved but have retained intracellular digestion in correlation with their mode of feeding,e.g.Brachiopoda, Rotifera, Tardigrada, Pyncogonida, Arachnida (other thanLimulus) and the majority of Mollusca excluding the Cephalopoda. These animals either feed on finely divided food (collected by ciliary mechanisms or scraped by a radula) or on fluid or semi‐fluid food which is sucked in.2In certain cases, notably the Lamellibranchia, but also in the Echinodermata, intracellular digestion is assisted or exclusively carried out by wandering phagocytic blood cells.3Extracellular digestion, originally developed with the increased size of available food as an aid to intracellular digestion, has completely replaced the more primitive form of digestion in certain rhabdocoel Turbellaria (probably), Polyzoa, Annelida, Myriapoda, Crustacea, Insecta, Cephalopoda and Chordata. This mode of digestion results in the reduction of the ingestive region of the gut and enables digestion, and the removal of indigestible material, to be hastened. The resultant increase in the rate of metabolism has had profound effects on the evolution of the Metazoa.4The appearance of extracellular digestion has been accompanied by changes in the structure and physiology of the gut. Distinct regions have been specialized for (1) the reception of food, (2) its conduction and storage, (3) digestion and internal triturition, (4) absorption, and (5) conduction and formation of faeces.5There is a definite correlation between the food of any animal and the nature and relative strengths of its digestive enzymes. Certain animals have acquired specific enzymes which enable them to exploit additional sources of food, the most important of such enzymes being cellulase and chitinase.6There is a periodicity of secretion in the digestive glands of many Metazoa,e.g.Gastropoda and Crustacea. In the Lamellibranchia and in style‐bearing Gastropoda, the style constitutes an ideal mechanism for the continuous liberation of small quantities of enzyme (amylase).7ThepH of the gut is controlled in various ways in different phyla. In ciliary‐feeding animals this may be of importance not only in securing the optimum conditions for the action of extracellular enzymes but also by its influence on the viscosity of the mucus with which the food is entangled.8There is evidence that the time taken for passage of food through the gut at any normal temperature corresponds to the period which is optimal for enzymatic action at that particular temperature.9The most successful groups of animals are (1) those which possess feeding and digestive mechanisms capable of utilizing, as a result of morphological and physiological adaptations, many types of food,e.g.Annelida, Crustacea, Insecta, Gastropoda and Vertebrata, and (2) those in which one type of food is collected and digested with great efficiency,e.g.Coelenterata, Turbellaria, Arachnida, and Cephalopoda (carnivorous); Brachiopoda, Lamellibranchia, and Tunicata (ciliary feeders); Trematoda and Cestoda (parasites). Of these, the first have been by far the more successful, owing to their capacity for exploiting new sources of food, in the invasion of new hab