Zoology Assignment Help >> Animal Kingdom, Body Organisation of Animals

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Animal Kingdom

Animals are acellular or multicellular organisms. Multicellular animals are heterotrophs. Approximately more than ten million species of living animals are known. These species differ from one another in shape, size, structure and ways of living. Animals are motile and are provided with sensory or nervous system. The only exception is porifers. They are devoid of proper nerve cells.

Body Organisation of Animals

Body organisation of animals can be studied under following heads-

1. Body plan-It can be further divided into three basic plans:

(a) Cell aggregate plan-Cells are loosely arranged in groups. The tissue or organ system is absent. e.g. porifers.

(b) The blind sac plan-It is more evolved plan than the cell aggregate plan. The alimentary canal has only single opening which opens outside. This is the opening for ingestion and egestion both. Division of labour is developed. This type of digestive tube is known as blind sac. e.g. coelenterates and flatworms.

(c) The tube within a tube plan¬ Digestive system is provided with mouth and anal opening. Ingestion by mouth and egestion by anus. More evolved and complex form.

2. Symmetry-It is divided as follows¬ –

(a) Asymmetrical-An animal which cannot be divided into two equal and similar halves from any point. Such symmetry is known as asymmetry. e.g. snail.

(b) Bilateral symmetry-When an animal is cut lengthwise in the middle vertical plane, then it is divided into two equal morror halves i.e. opposite right and left halves. This symmetry may not be internal symmetry also, e.g. frog, man, earthworms, insects etc. (most of the animals).

(c) Radial symmetry-When an animal is divided through its longitudinal axis through any diameter, then body will have two equal halves, e.g. hydra, star fish etc.

3. Body cavity or Coelom-The body cavity or coelom is present only in the animals with tube within a tube body plan. Their cavity is filled with the fluid. This is known as body coelom.

This coelom gives flexibility to the body. All the internal organs are suspended in the coelom. Thus coelom protects and acts as a shock absorber.

Except in roundworms, the coelom develops from embryonic mesoderm in all animals. Porifers, coelenterates and platyhelminthes are devoid of coelom, hence also known as acoelomates.

4. Body segmentation-The body is divided into constricted body segments. These segmentation arises in the annelids and is also present in the arthopods and all chordates.

5. Body support and protection¬ Soft bodied animals are provided with exoskeleton. e.g. foramenifera (protozoans), coelenterates, arthropods and most of the molluscans. The exoskeleton is formed by the deposition of hard Substances or shell. The exoskeleton is also present in the vertebrates such as scales, feathers, hairs, nails etc.

Hard, Hard, bony and cartilaginous tissue forms the exoskeleton of the body especially of vertebrates. Notochord supports the body of primitive adult chordates whereas vertebral column is a supporting skeleton of vertebrates. Appendicular skelecton provides a mean of locomotion and brain case (cranium) and vertebral column protects the central nervous system in vertebrates. The axial skeleton gives the length or height to the animals.

Classification of Animal Kingdom

Invertebrate phyla-Animals devoid of notochord or vertebral column are known as invertebrates. There are about 30 phyla of invertebrate group and every phylum is provided with entirely different characteristics. Evolutionary studies of animals of same phylum have directly or indirectly originated from a single common ancestor.
Lower and higher invertebrates¬-The lower invertebrates are provided with simple body organisation and are generally small in body size. It includes protozoa, porifera, coelenterata, platy helminthes and nematoda.

The higher invertebrates are generally larger bodied animals and their body organisation is also complex. It includes annelida, arthropoda, mollusca and echinodermata.