Classification of the Animals
The animal kingdom is divided into two sub-kingdoms: Protozoa and Metazoa. Protozoa are unicellular or acellular animals, that is, the individuals have a body not sub-divided into cells. Metazoa are cellular animals in which the body is divided into many cells. These cells may be arranged into tissues and organs.
The sub-kingdom Metazoa is further sub-divided into two branches: Parazoa and Eumetazoa. The Parazoa or Spongiaria comprises the sponges in which the cells are loosely aggregated without any digestive tract or mouth. No well organised tissues and organs are met within this group of animals. Eumetazoa consists of animals in which the cells are organised into tissues and organs. A nervous system and a digestive tract with or without an anus are also found in these animals.
Within the eumetazoa, a few animals have an organisation of two layers of cells known as the ectoderm and the endoderm, with a jelly like intervening layer called the mesoglea. They are radially symmetrical animals with a single space called the gastrovascular cavity which is also the digestive tract. These animals form the division Radiata.
The eumetazoa other than radiata have their organs derived from three cell layers known as the ectoderm, the endoderm and the mesoderm in between. These animals are bilaterally symmetrical and have a digestive tract with or without an anus. They constitute the division Bilateria.
The division Bilateria shows three grades of organization:
- The space between the digestive tract and the body wall is filled with a tissue known as the parenchyma derived from the mesoderm (Acoelomata).
- The space between the digestive tract and the body wall encloses a cavity which is not lined with epithelial cells. This cavity is the remnant of a cavity known as the blastocoel which appeared at an early stage in the development of animals and is termed pseudocoel. Animals that have pseudocoel in the adult stage are called Pseudocoelomate.
- A body cavity is present between the digestive tract and the body wall, which is lined with epithelial cells derived from the mesoderm. This cavity is the true coelom, and the animals possessing it are known as Eucoelomata.
Although zoologists differ in the number of phyla that constitute the animal kingdom, at present, over two million animals are confined to about 35 phyla. For the sake of convenience the entire animal kingdom can be divided into two groups: Invertebrates and Chordates. Animals which do not possess a vertebral column or backbone are called invertebrates; protozoans, flatworms, earthworms, insects, molluscs and starfishes all belong to this group. Chordates have a backbone consisting of either a cartilaginous notochord (chorda dorsalis) or a vertebral column consisting of a number of bony vertebrae; fishes, frogs, toads, lizards, snakes, birds and mammals including man are all vertebrates possessing a vertebral column. The general characters and classification of invertebrates and chordates are detailed in the following two chapters.