Cellular Level: In case of cellular level organization, a single cell is responsible for all the metabolic activities. Cellular level organization is present in unicellular animals and some of the muclticellular animals.
Tissue Level: In case of tissue level organization, a group of cells is responsible for all the metabolic activities, e.g. coelenterates.
Organ Level: In case of organ level organization, some specialized organs are present for some specific functions, e.g. Platyhelminthes.
Organ System Level: In organ system level organization, complex organ systems are present for various functions, e.g. mollusca, chordate.
Some of the animals are almost asymmetrical. Their body cannot be divided into two equal halves from any plane, e.g. sponges.
Radial Symmetry: In case of radial symmetry, any plane passing through the central axis divides the body into two identical halves, e.g. coelenterates, ctenophores, echinoderms, etc.
Bilateral Symmetry: In case of bilateral symmetry, the body can be divided into two identical halves only through a single plane, e.g. annelida, arthropoda, etc.
When the cells are arranged in two embryonic layers, the animal is called a diploblastic animal. The two layers are; ectoderm and endoderm. Mesoglea; which is an undifferentiated layer is present between the ectoderm and endoderm. Example: coelenterates.
When the cells are arranged in three embryonic layers, the animal is called triploblastic animal. The three layers are; ectoderm, mesoderm and endoderm. Examples: Platyhelminthes to chordates.
The body cavity; which is lined by mesoderm is called coelom.
Coelomates: If coelom is present, the animal is called coelomate, e.g. annelids, molluscs, arthropods, echnioderms, hemichordates and chordates.
Pseudoceolomates: If the body cavity is not lined by mesoderm but the mesoderm is present as scattered pouches in between the ectoderm and endoderm, the animal is called pseudocoelomate, e.g. aschelminthes.
Acoelomates: When the body cavity is absent, the animal is called acoelomate, e.g. platyhelminthes.
The body of some animals is externally and internally divided into segments with serial repetition of at least some organs. For example; the body of the earthworm shows metameric segmentation. This phenomenon is called metamerism.
Notochord is a mesodermally derived rod-like structure. It is formed on the dorsal side during embryonic development in some animals. If notochord is present then the animal comes under chordates. An animal without notochord is called non-chordate, e.g. porifera to echinoderms.
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