Animal kingdom – Classification
The kingdom is the second-highest taxonomic rank below the domain. Under the kingdom Animalia, the multicellular, eukaryotic organisms with few exceptions are placed.
Basis of Classification
Some common features are used as the basis of the classification of animals. These are common to many individuals related to the arrangement of the cell- level of organization, the symmetry of the body, the nature of coelom (body cavity), patterns of the digestive system, the circulatory or reproductive system.
The Level of Organisation
We know that all members of Animalia are multicellular, but all of them do not possess the same pattern of body organisation of cells. Some organisms have cellular levels, some tissue level, organ and organ system levels.
- As in Sponges, the cellular level of organisation is found.
- In Coelenterates, tissue level of organisation is found.
- Organ level of organisation is found from Platyhelminthes and other higher phyla.
- The organ system level of Organisation is found in animals like Annelids, Arthropods, Molluscs, Echinoderms, and Chordates.
Organ systems show variation in pattern complexities in different animals.
- Incomplete digestive system: As we can see the digestive system in Platyhelminthes consists of a single opening outside of the body, functions for both mouth and anus, so it is called incomplete.
- The complete digestive system: In the complete digestive system two openings i.e. mouth and anus are found.
The circulatory system may be of two types i.e. Open type and Closed type.
- Open type of circulatory system: The blood is pumped out of the heart and the direct bath of cells and tissue takes place.
- Closed type of circulatory system: In this type of circulation, the blood circulates through a series of vessels like arteries, veins, and capillaries.
The body may or may not be divided into two equal and identical parts. On the basis of these features, the animal can be categorized as:
- Asymmetrical– Any plane that passes through the center does not divide into equal parts. Ex. Sponges found mostly asymmetrical.
- Radial symmetry– In this type, any plane that passes through the axis of the center and the body divides into two equal halves, then it is called radial symmetry. For example- Coelenterates, Ctenophores, and Echinoderms show this type of symmetry.
- Bilateral symmetry– the body divides into identical left and right planes in one plane only then it is called bilateral symmetry. For example-Annelids, Arthropods, etc show this type of symmetry.
The Diploblastic and Triploblastic Organisation
- Diploblastic animals– The cells found arranged in two embryonic layers i.e. external ectoderm and an internal endoderm. Mesoglea is an undifferentiated layer found between ectoderm and endoderm. Example- Coelenterates
- Triploblastic animals– In a developing embryo, the third germinal layer called mesoderm is found in between ectoderm and endoderm. These animals are called triploblastic animals. These are Platyhelminthes to chordates.
Coelom or Body cavity
- Coelomates– The coelom is the body cavity lined by a mesoderm. The animals consisting of coelom are called coelomates. Example- Annelids, molluscs, arthropods, and chordates.
- Pseudocoelomates– The body cavity is not lined by mesoderm in some animals, but mesoderm is present as scattered pouches in between ectoderm and endoderm. This type of body cavity called pseudocoelom and the organism is called pseudocoelomate. As in aschelminthes.
- Acoelomate– In these organisms, the body cavity is not found. As in Platyhelminthes.
The body of some organisms is externally and internally divided into segments with synchronized repetition of some organs. Such as in earthworms, the body shows a pattern which is termed metameric segmentation, and the phenomenon is called metamerism.
On the basis of Notochord
It is a mesodermally derived structure found in a rod-like shape on the dorsal side during embryonic development. Animals with notochord are called Chordates and without notochord are called non- chordates. Porifera to Echinodermata are non-chordates.
The major phyla are: