Basis Of Classification
There are various criteria on which the Animal Kingdom is classified. Some of them are levels of organization, symmetry, germinal layers, presence of coelom or body cavity, segmentation and notochord.
Levels of Organisation
All animals are multicellular. But different animals have different levels of organization of cells. While some animals are simple aggregation of cells, some others are complex arrangement of cells resulting in formation of tissues and organs.
In this case, each cell of the animal is responsible for all the life processes, like respiration, nutrition, excretion, etc. Cellular level organization is present in all the poriferans, i.e. sponges.
You must be aware that a group of cells responsible for specific function is called tissue. In case of tissue level organization, a group of cells is responsible for a particular metabolic activity. There can be a tissue responsible for digestion, another tissue responsible for respiration and third one for excretion. Tissue level organization is present in coelenterates and ctenophores.
In case of organ level organization, some specialized organs are present for some specific functions. An organ may be responsible for digestion, another for respiration and a third one for excretion. Organ level organization is seen in platyhelminthes and aschelminthes, i.e. in roundworms and flatworms.
Organ System Level
In organ system level organization, complex organ systems are present for various functions. You have seen that for a particular function there is a single organ in case of organ level organization. But in case of organ system level organization, there is more than one organ to carry out a specific function. Let us take the example of human digestive system to understand this. There are many organs in human digestive system, like stomach, intestines, liver, pancreas, etc. All of them carry out different aspects of digestion and are collectively responsible for digestion of food. Organ system level of organization is present in animals from Annelida to Chordata.
Symmetry means if a figure can be divided into two equal halves or not. Animals can be asymmetrical, bilaterally symmetrical or with radial symmetry.
Some of the animals are almost asymmetrical. Their body cannot be divided into two equal halves from any plane. Most of the sponges are asymmetrical.
In case of radial symmetry, any plane passing through the central axis divides the body into two identical halves. In this case, the animal body appears circular when viewed from top or bottom. Radial symmetry is seen in coelenterates, ctenophores, echinoderms, etc.
In case of bilateral symmetry, the body can be divided into two identical halves through a single plane, only. Bilateral symmetry is seen in annelida, arthropoda, mollusca, echinodermata, hemichordata and chordata. It is important to remember that based on the stage of life, echinodermates can show radial symmetry or bilateral symmetry.
Germinal layers are the layers of cells in embryo. These are called germinal layers because different organs develop or Germinate from different layers. Some organs may germinate from endoderm, some others from mesoderm and some from ectoderm.
When the cells are arranged in two embryonic layers, the animal is called a diploblastic animal. The two layers are ectoderm and endoderm. An undifferentiated layer is present between ectoderm and mesoderm. This layer is called mesoglea. 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.
Body cavity or coelom can be present or absent in different animals. The body cavity is lined by mesoderm. Based on this factor, animals are divided into three groups, viz. coelomates, pseudocoleomates and acoelomates.
If coelom is present, the animal is called coelomate, e.g. annelids, molluscs, arthropods, echnioderms, hemichordates and chordates.
In this case, the mesoderm does not make the lining of body cavity rather the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Aschelminthes are examples of psudocoelomates.
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. The term metamere means segments. You may be having a question in mind. Arthrodpods also have segmented body and their body is divided into three segments, viz. head, thorax and abdomen. How are arthropods different from annelids? In case of arthropods, the segmentation is external only. This means there is no internal segmentation and there is no serial repetition of organs in arthropods. But the body of an earthworm is segmented both externally as well as internally, and there is serial repetition of some organs, like nerve ganglions.
Notochord is a rod-like structure, present on the dorsal side of the animal. Notochord is mesodermally derived, which means it develops from mesoderm of the embryo. Notochord appears during embryonic development and disappears later on in most of the animals. In some animals, it may be present as rudiment even at later stages. Our vertebral column was a notochord during embryonic stage. If notochord is present then the animal comes under chordates. An animal without notochord is called non-chordate, e.g. porifera to echinoderms. Hemichordates and chordates are animals with notochord.
- Cellular Level Mostly asymmetrical, acoelomate, e.g. Porifera
- Tissue/Organ Level
- Radial Symmetry
Acoelomate, e.g. Coelenterate, Ctenophora
- Bilateral Symmetry
Annelida, Arthtropoda, Mollusca, Echinodermata, Hemichordata, Chordata
- Radial Symmetry