With over a million animal species on our earth, it would be difficult to categorize them into different groups. However, with the help of basic fundamental features such as level of organisation, symmetry, cell organisation (body wall), nature of coelom, segmentation and notochord, scientists have classified animals into various groups.
- Levels of Organisation
1. 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 multicellular animals.
2. Tissue Level: In case of tissue level organization, a group of cells is responsible for all the metabolic activities, e.g. coelenterates.
3. Organ Level: In case of organ level organization, some specialized organs are present for some specific functions, e.g. Platyhelminthes.
4. 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.
1.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.
2.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.
- Cell organisation
Cell organisation is the next feature used for classification.The body wall of the animal may be arranged in two or three embryonic layers.There are two types of cell organisation–diploblastic and triploblastic.
1.Diploblastic: 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.
2.Triploblastic: 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.
- Nature of coelom
The coelom is the body cavity lined by mesoderm. The presence or absence of this cavity helps in classifying the animals. There are three categories, coelomates, acoelomates and pseudocoelomates.
1.Coelomates : They are the animals that possess a body cavity. They include annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
2.Acoelomates: They are the animals in which the body cavity is absent. They include organisms belonging to platyhelminthes.
3.Pseudocoelomates : They are the animals that possess a pseudocoelom, such as aschelminthes. A pseudocoelom is a body cavity in which the mesoderm is not lined, but rather scattered between the ectoderm and the endoderm.
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