Method of Animal Classification
The inherent differences and similarities exhibited by animals form the basis of classification of animals. For the purpose of classification, characters that show homology should be distinguished from those that exhibit analogy. The forelimbs of frogs, lizards, birds, dogs and bats have the same ground-plan of structure and show similarity of origin. These structures, however, are functionally different. Organs of such structural agreement and origin are called homologous organs and are of immense value in classifying animals. Homologous structures indicate a common ancestry. The wings of a fly and of a bird are similar in function, but differ in structure; they have arisen independently to suit the purpose of flying. Such structures that show functional similarity but are different structurally, are called analogous structures. These structures have no significance in classification. The classification based on homologies of organs show how different kinds of animals are derived from ancestral types by the modifications of organs to suit a particular mode of life. In this way the evolutionary connection of different groups of animals are brought out, as in a genealogical tree. This method of classification is known as natural classification. The classification based on the habits and habitats of animals cannot bring out the evolutionary relationship between the animals and are, therefore, said to be artificial. According to natural classification different groups of animals are interrelated as if by blood relationships.
Some General Features of Animals
There are certain basic characteristics of animals which help in establishing the evolutionary interrelationships among different groups of animals. These are:
- grades of organisation,
- types of symmetry, and
- kinds of body cavity.
Grades of Organisation and Body PlanThis refers to the organisation of cells in an animal whether single-celled or many-celled.
Types of Symmetry
Symmetry may be defined as the arrangement of parts in relation to points, planes and straight lines. Multicellular animals have two types of symmetry. These are radial symmetry and bilateral symmetry.
- Radial Symmetry: The principal parts of the body are arranged in a radiating fashion around a principal axis. This is characteristic of coelenterates and ctenophores.
- Bilateral Symmetry: All the higher multicellular animals are bilaterally symmetrical in which the animal body can be divided into two mirror halves through a median longitudinal plane.