In some animals, the body is of many segments, which show serial repetition of parts, for example, earthworm. This kind of segmentation is called metameric segmentation and the phenomenon is known as metamerism.
Body Cavity or Coelom
The presence (eucoelomate) or absence (pseudocoelomate and acoelomate) of a body cavity is another characteristic that helps in the classification of animals.
Animals which posses a coelom or body cavity are called eucoelomates. In eucoelomates there are two methods by which the coelomic cavity is formed. Coelom is any cavity which is bounded by mesoderm. In certain animals coelomic cavity is formed as mesodermal pouches from the gut. This sort of coeolomic cavity is called enteroceolic coelom. However, in higher invertebrates as well as chordates the coelom is formed in different ways.
During embryogenesis the mesoderm accumulates as solid blocks of cells on either side of the notochord. These mesodermal blocks are called somites. During further development somites undergo differentiation dorsoventrally into epimere, mesomere (nephorotome) and lateral plate mesoderm. The lateral plate mesoderm on either side of the gut of the embryo grows ventrally and meets each other. A split appears in the lateral plate mesodermal sheet so that it is split into an outer somatic layer and an inner splanchnic layer with a cavity in between. This cavity, which is called the splanchnocoel, is the rudiment of the body cavity or coelom. This sort of coelom is called scizocelic coelom.
Heterotrophic Mode of Nutrition
Unlike plants, which synthesize their own food and are called autotrophs, animals that feed upon other living forms are called heterotrophs. The heterotrophs may be herbivores (eating only plants), carnivores (eating only animals) and omnivores (eating both animals and plants). There are animals that depend on other organisms for food. These are called parasites and the relationship is known as host parasite relationship.
Compared to members of other kingdoms, animals perform a more rapid and complex way of movement. Some animals can fly (butterflies, bats and birds); Some can swim (jellyfish, squids, fishes, whales) and others can run or walk on land. Movement of animals is directly related to the flexibility of their cells, and is perhaps, the most striking characteristic of animals.
Most animals reproduce sexually. Cells formed after meiosis function directly as gametes. Male produces male gametes (spermatozoa), and female produces female gametes (ova). Fusion of gametes (fertilization) results in the formation of a zygote. As a result of sequential development, zygote produces a complete organism.