Animals are those organisms which are eukaryotic, multicellular and heterotrophic in their mode of nutrition. Except few, most animals are mobile and do not have cell wall.
Multicellular animals are called Metazoa which have been divided into two branches: Parazoa and Eumetazoa.
- In Parazoa, the body of animal is made up of loose cells, have poorly differentiated tissues and have no organ or digestive cavity. E.g sponges.
- In Eumetazoa the cells are properly arranged into tissues and organs. It is further classified on the basis of (a) the number of germ layers present in the embryo; (b) the symmetry of the body of the organism; and (c) mode of origin of mouth. Those animals which have two germ layers in the embryo are called diploblastic animals and those having three germ layers are called triploblastic animals.
In diploblastic animals, the outer cell layer is called ectoderm, while the inner layer is called endoderm and both of these germ layers enclose a non- cellular jelly-like layer, the mesoglea. The triploblastic animals have three germ layers, namely ectoderm, mesoderm and endoderm.
The two sides of the body are mirror images of one another. Such body symmetry type is called bilateral symmetry. Some animals have radial body symmetry i.e having similar parts in a regular pattern around a central axis.
Two types of animals have been recognised on the basis of origin of mouths: Protostomia (mouth arising from or near the blastopore of gastrula; gastrula is an embryonic stage) and Deuterostomia (mouth arising anteriorly at some distance from the blastopore).
Further, based on the presence or absence of body cavity or coelom, animals are groups into acoelomata, pseudocoelomata and coelomata. The acoelomates do not have body cavity because the space between the body wall and digestive tract is filled with parenchymatous tissue. Also, these tissues have a cavity between the body wall and digestive tract but this does not arise from the mesoderm and is not lined by epithelial cells. The Coelomates have true body cavity which originates from mesoderm and is lined by epithelial cells of mesodermal origin.
For further understanding refer below:
Examples of main phyla of kingdom Animalia are as follows:
- Phylum 1 Porifera (porus- pore, ferre- to bear; ‘organisms with holes’)E.g: Sycon, Euplectella (Venu’s flower basket), Hyalonema (rope sponge), Spongillia, Euspongia ( Bath sponge) etc.
- Phylum 2 Cnidaria or Coelenterata ( koilos- hollow, enteron- gut)E.g: Hydra, Millepora (coral), Aurelia (jelly- fish), Rhizostoma, Fungia (mushroom coral) etc.
- Phylum 3 Ctenophora (ktenos-comb, phora- carrying)E.g: Pleurobrachia (comb jelly), Cestum (Venu’s girdle), etc.
- Phylum 4 Platyhelminthes (platys-flat, helmins- worm; Flatworms) E.g: Planaria, Fasciola,Schistosoma (blood- fluke), Taenia solium (pork tape worm) etc.
- Phylum 5 Nematoda E.g: Ascaris (round- worm), Wucheria bancrofti (filarial worm), Enterobius (pinworm of human) etc.
- Phylum 6 Annelida ( annelus- a ring; segmented worms ) E.g: Nereis (sand worm), Aphrodite (sea mouse), Pheretima (earthworm), Hirudinaria (Indian cattle leech) etc.
- Phylum7 Arthropoda (arthros- jointed; podos- foot; jointed- legged animals) E.g: Peripatus, Cancer (true cab), Scolopendra (Centipede), Anopheles (mosquito), Pieries (butterfly) etc.
- Phylum 8 Mollusca (molluscus- soft) E.g: Pila (apple nail), Helix (garden snail), Octopus (devil-fish) etc.
- Phyllum 9 Echinodermata ( echinos- spiny or hedge hog, derma- skin; spiny skinned animals) E.g: Antedon (feather star), Cucumaria (sea cucumber), Asterias (star- fish or sea star) etc.
- Phyllum 10 Chordata ( chorda- string) It is the most advanced goup. It consists of reptiles, Amphibians, Aves, Mammalia etc.
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