The major group of animals are classified under the Kingdom Animalia, also known as Metazoa. This kingdom does not contain prokaryotes. All the members of this kingdom are multicellular, eukaryotes. They are heterotrophs, they depend on other organisms directly or indirectly for food. Most of the animals ingest food and digest in the internal cavity. Most of the organisms are motile which means they can move independently and spontaneously.
There are around 9 to 10 million species of animals, and about 800,000 species are identified. Fossil records of animals were found in the era of the Cambrian explosion, about 540 million years ago. Animals are divided into various sub-groups, biologists have identified about 36 phyla within the animal kingdom including birds, mammals, reptiles, fish, amphibians etc.
The word 'animal' is derived from the Latin word animalis which means 'having breath'. The Kingdom Animalia is characterized by eukaryotic and heterotrophic organisms. They are multicellular and lack cell wall. They depend directly or indirectly of plants for their food. Food is ingested and digested in their internal cavity and food reserves are stored as glycogen or fat. Nutrition is holozoic, i.e., by ingestion of food. Animals follow a definite growth pattern, the adults have a definite shape and size. Higher forms of animals exhibit well developed sensory and neuromotor mechanism. Most of the organisms are capable of locomotion. Reproduction is by copulation of male and female which is followed by development in embryonic stages.
General characteristics of the Kingdom Animalia are as follows:
- Animals are eukaryotic, multicellular and heterotrophic organisms.
- They have multiple cells with mitochondria and they depend on other organisms for food.
- Habitat - Most of the animals inhabit seas, fewer are seen in fresh water and even fewer on land.
- There are around 9 to 10 million animal species that inhabit the earth. Only 800,000 species are identified.
- Biologists recognize 36 phyla in the animals kingdom.
- Size - The sizes of animals ranges from a few celled organism like the mesozoans to animals weighing many tons like the blue whale.
- Animal bodies - Bodies of animals are made of cells organized into tissues which perform specific functions. in most animals tissue are organized into complex organs, which form organ systems.
- Cell structure - The animal cell contains organelles like the nucleus, mitochondria, Golgi complex, ribosomes, endoplasmic reticulum, lysosomes, vacuoles, centrioles, cytoskeleton.
- Animals are made up of many organ systems, that aids in performing specific functions that are necessary for the survival of the organism.
- Organ systems are skeletal system, muscular system, digestive system, respiratory system, circulatory system, excretory system, reproductive system, immune system and the endocrine system.
- Body symmetry - Most of the animals are bilaterally symmetrical, while primitive animals are asymmetrical and cnidarians and echinoderms are radially symmetrical.
- Locomotion - Most animals have the ability to move, they show rapid movement when compared to plants and other organisms.
- Respiration - It is a gaseous exchange of taking in oxygen and giving out carbon dioxide. This process takes place in organs of respiration like the lungs, gills, book gills and book lungs and some animals skin is also used for respiration.
- Digestion - Animals ingest food, and digestion takes place in the internal cavity like the digestive system in animals, in primitive animals vacuoles are for digestion.
- Nervous system - Sensory mechanism and the coordination of the organ systems is carried on by the nervous system. In animals the nervous system comprises of nerve ganglions, or brain, spinal cords and nerves.
- Circulatory system - The distribution of nutrients, exchange of gases and removal of wastes takes place in the circulatory system. This system comprises of the heart, blood vessels and the blood.
- Excretory system - Removal of wastes from kidneys.
- Skeletal system - support and protection is provided by the skeletal system.
- Reproductive system - Most animals reproduce sexually, by the fusion of haploid cells like the eggs and the sperms.
- Glands of the endocrine system help in control and coordination of the body system.
Kingdom Animalia has approximately 36 sub-divisions known as 'phyla'. Each phyla share particular properties structurally and functionally which together separate it from other phyla. Below are the most common phyla classified under traditional biological methodology. Phylum Porifera
- They are primitive organisms, most of them are salt-water sponges. They do not have organs or nerve cells or muscle cells. Approximately, 8,000 species exist today. Example: Sycon, Euspongia, Spongilla.Phylum Coelentrata (Cnidaria)
- This group is composed of jelly-fish and other lower aquatic animals. Approximately, 15,000 species exist today.Example: Aurelia, Adamsia
- This group consists of flat worms. They inhabit both marine and fresh water habitats and they are mostly endoparasites found in animals. Example: Taenia, Fascicola
- It is a group of round worms, most of them are parasites. This phylum consists of about 80,000 parasitic worms.
- They are present in aquatic, terrestrial and are free-living or parasitic in nature. This phylum comprises of segmented worms. Example: Earthworm, Leech etc.Phylum Arthropoda
- This is the largest phylum which consists of insects. There are over 1 million species of insects existing today. Example: Locusts, Butterfly, Scorpion, Prawn.
- It is the second largest phylum. They are terrestrial and aquatic. Example: Pila, Octopus
- This consists of sea stars and sea urchins. There are about 6,000 species. Example: Asteria
- Animals of this phylum have a characteristic feature of presence of notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. Within this phylum advanced group called vertebrates which include fish, amphibians, reptiles, birds and mammals.
Some of the well known examples of Kingdom Animalia
Phylum Porifera - Sycon, Euspongia, Spongilla
Phylum Coelentrata - Aurelia, Adamsia.
Phylum Platyhelminthes - Taenia
Phylum Aschelminthes - Ascaris
(Round worm), Wuchereria
(Filaria worm), Ancylostoma
Phylum Annelida - Nereis
(blood sucking leech).
Phylum Arthropoda - Apis
(honey bee), Bombyx
(lac insect), Anopheles
Phylum Mollusca - Pila
(Apple snail), Octopus
(devil fish), Pinctada
(pearl oyster), Aplysia
(Tusk Shell), Chaetopleura
Phylum Echinodermata - Ascarias
(star fish), Echinus
(sea urchin), Antedon
(Sea lily), Cucumaria
(sea cucumber), Ophiura
Phylum Chordata - Balanoglossus
. All fishes, amphibians, reptiles, birds and mammals.