Criteria for Classification of Animal Kingdom

Criteria for Classification of Animal Kingdom

Look at this list of animals: sponge, rotifer, jelly fish, flatworm, roundworm, snail, earthworm, grasshopper, star fish and peacock.

Among the above listed animals, sponge does not have any true tissues. We can divide the animalia into two major divisions - those with true tissues and those without true tissues. 

The group of animals that lack true tissues are called as Porifera.

It is seen that the jelly fish and star fish have radial symmetry, while if we look at flatworm, roundworm, rotifer, snail, earthworm, grasshopper and peacock have bilateral symmetry.

1.     Grade of organization – Animals are grouped as unicellular or multicellular based on the number of cells.

2.     Symmetry – It is a plane of arrangement of body parts. Radial symmetry and bilateral symmetry are the two types of symmetry (Figure 3). In radial symmetry the body parts are arranged around the central axis, if we cut through the central axis in any direction, it can be divided into similar halves. E.g. Hydra, jelly sh and star sh. In bilateral symmetry, the body parts are arranged along a central axis, if we cut through the central axis, we get two identical halves E.g. Frog.

3.     Germ layers – Germ layers are formed during the development of an embryo. These layers give rise to different organs, as the embryo becomes an adult. If an organism has two germ layers, the ectoderm and the endoderm it is said to be diploblastic. If they have three germ layers, the ectoderm, the mesoderm and the endoderm they are triploblastic animals.

4.     Coelom – Coelom refers to a fluid-filled cavity inside the body. It separates the digestive tract and other organs from the body wall. A true body cavity or coelom is one that is located within the mesoderm. Based on the nature of the coelom, animals are divided into 3 groups (Figure 4). Organisms like the earthworm are called coelomates or eucoelomates because they have true coelom. Tapeworm is an example of an acoelomate because it does not have a body cavity. Animals like the roundworm have a body cavity but it is located between the endoderm and the mesoderm.

This is considered to be a false coelom and these organisms are called pseudocoelomates.

Characters like presence or absence of body cavity (coelom), segmentation, exoskeleton, jointed legs (appendages), notochord are used to classify the animalia into ten major Phyla (Figure 5).

Kingdom Animalia is divided into two groups, based on the presence or absence of notochord Invertebrata and Chordata (Prochordata and Vertebrata). The groups invertebrata is classi ed as follows