Animals

An Animal is a Living Organism characterized by voluntary movement. Animals are a diverse group of Organisms inhabiting various types of Habitats like Forests, Lakes, Mountains etc. Animals are generally considered to have evolved from a Flagellated Eukaryote (type of complex cell). Their closest known living relatives are Complex Cells like the Choanoflagellates, Collared Flagellates that have a morphology similar to the Choanocytes of certain Sponges. Molecular studies place Animals in Supergroup called the Opisthokonts, which also include the Coanoflagellates, Fungi and a few small Parasitic Protists. The name 'Opisthokonts' comes from the Posterior location of the Flagellum (whip like structure) in Motile Cells, such as most Animal Spermatozoa, whereas other Eukaryotes tend to have Anterior Flagella.

Chracteristics of Animals

1. Multicellular - Animals' bodies are made up of Multiple Cells, with the exception of Sponges. In most Animals, these Cells are organized into different Tissues that perform different functions.

2. Eukaryotic - All Animals are Eukaryotes, which means that, they are made up of Complex Cell that have Membrane-Bound Nuclei and Organelles. Their DNA is linear and organized into Chromosomes.

3. Heterotrophic - All Animals are Heterotrophic, meaning that they require organic compounds of carbon and nitrogen for nourishment. They feed on other Organisms, directly or indirectly. They eat other Organisms and thus obtain Carbon from the organic materials that make up the other Organisms. They generally digest food in an internal chamber, which separates them from Plants and Algae.

4. Motility - Motility means to moves spontaneoulsy and swiftly. Unlike plants, which are fixed to the substrate in which they grow, most Animals are Motile. One exception is Sponges, which are Sedentary Organisms.

5. Blastula Stage - In most Animals, Embryos pass through a Blastula Stage, which is a characteristic exclusive to Animals. Most Animals reproduce sexually via egg and sperm. This means genetic information is crossed between individuals and variability between parents and offspring is maintained. There are a few animals that are capable of asexual (non sexual) reproduction, like Corals and Sponges.

6. Diploid - Most animals are made up of Diploid Cells. In Diploid Cells, there are two full sets of the Cell's genetic material.

7. Absence of Rigid Cell Walls - Animals do not possess Rigid Cell Walls unlike Plants, who have Rigid Cell Walls.

Types / Classification of Animals

Animals are generally Classified into Two Major Categories:

1. Vertebrates
2. Invertebrates

Scientists estimate that there may be between 3 and 30 million Species of Animals on our Planet.

Of these Species, about 97% are Invertebrates and 3% are Vertebrates.