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As described in the previous section, the animal kingdom is classified based on the body symmetry, arrangement of cells, germinal layer, nature of coelom, segmentation, and presence of notochord. The nature of coelom, segmentation, and presence of notochord are explained briefly in this section.
Nature of coelom:
Between the body wall and the gut wall, lies a cavity called as coelom. The body cavity which is lined by mesoderm is called as coelom. The presence or absence of coelom is a characteristic feature used for classification purpose. Animals like Platyhelminthes which lack the presence of coelom in the body cavity are known as acoelomates. Animals like annelids, mollusks, echinoderms, hemichordates, and chordates possess coelom and hence are known as coelomates. In some animals, the mesoderm is scattered between endoderm and ectoderm. Such body cavity id called pseudo coelom and the animals possessing this body cavity are known as pseudocoelomates. For example, helminths.
In some animals, the body is internally or externally divided into various segments with a serial repetition of some organs. Animals like an earthworm, tapeworms show metameric segmentation and this phenomenon is called as metamerism.
The notochord is defined as a rod-like structure present on the dorsal side of the body of the animal. The notochord is derived from mesodermal germinal layer during embryonic development. Organisms belonging to class Porifera to echinoderms lack the presence of notochord and hence are known as non-chordates. Higher level organisms consist of the notochord and hence are known as chordates.
Classification of Animal Kingdom:
The Animal Kingdom is broadly classified into different phyla based on the level of cell organization, body symmetry, and nature of coelom.