 Dear students, in this topic, we shall take an overview of the elementary systems found in animals. The elementary systems are involved in procuring food and converting it into a usable form by the cells of the animal body. Both these processes of procuring and digesting or converting it are known as nutrition. The major steps of animal nutrition include digestion, digestion of the ingested food products. Then absorption of digested food products are products of digestion. Number three, the digestion are removal of indigestible parts of the food and undigested waste products. There are two major types of elementary systems which are observed in all animal kingdom. The first elementary system is a simple sack like gastro vascular cavity, while the second type of elementary system which is found in most of the animals is a tubular elementary canal also known as the gut. Dear students, the simple sack like gastro vascular cavity is the characteristic feature of Nideria. It has a single opening which is called mouth. This mouth serves for the ingestion or intake of food as well as it serves for the ingestion or removal of indigestible wastes. The tubular elementary canal is found in complex bilateral animals. This tube is a hollow tubular cavity which is open at both ends. The first opening is called mouth which is for ingestion or intake of food, while the second opening is called anus which is for ingestion. Dear students, the elementary canal or complete digestive tract has certain advantages for the animals. It increases the efficiency of digestive and absorptive processes. It also allows unidirectional flow of food contents through the digestive tract. This type of elementary canal or elementary system also allows regional specialization for different types of digestive actions. Dear students, in the regional specialization elementary canal this helps in various functions related to digestion that is mechanical, chemical and bacterial treatment of food in special regions of the elementary canal. The second major function is absorption of products of digestion and the storage of undigested waste materials is carried out or takes place in special regions of the elementary canal. This is known as regional specialization.