Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., Coelenterates.Animals like Annelids, Arthropods, etc., where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.ĭiploblastic and Triploblastic Organisation.Coelenterates, Ctenophores and Echinoderms have this kind of body plan. When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry.Sponges are mostly asymmetrical, i.e., any plane that passes through the centre does not divide them into equal halves.Animals can be categorised on the basis of their symmetry.Similarly, the circulatory system may be of two types: open type in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it and closed type in which the blood is circulated through a series of vessels of varying diameters (arteries, veins and capillaries).A complete digestive system has two openings, mouth and anus. For example, the digestive system in Platyhelminthes (incomplete digestive system) has only a single opening to the outside of the body that serves as both mouth and anus, and is hence called incomplete.
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