Animals (Animalia) are a large group of multicellular, eukaryotic, heterotrophic organisms, which is classified as a kingdom in biology. Animals swallow food and digest it inside the body. Most animals are motile organisms. They are included in the domain of Eukaryotes. Aristotle was the first to classify the entire living world into animals and plants. This original division has remained in science for a long time, but due to the accumulation of new information and facts during the last centuries, the division of organisms has become more complicated. The modern classification of organisms follows the existence of a hierarchy of groups (taxonomic categories), where the animal kingdom is among the most general. The number of empires varies greatly in the understandings of different authors, the number 5 is most often mentioned (Whittaker), or, much more realistically, the number of about 100.
When talking about the animal world, the word fauna is often used.
Origin of the word "animal"
The word "animal" comes from the Latin word Animalis, which means "has a breath". In everyday colloquial use, the word refers to non-human members of the Animalia kingdom. Sometimes, only close relatives of humans such as mammals and other vertebrates are intended for colloquial use. The biological definition of the word refers to all members of the Animalia kingdom, encompassing creatures as diverse as sponges, jellyfish, insects and humans.
The animal kingdom includes heterotrophic multicellular organisms, whose cells are organized at least into primitive tissues. These organisms are widespread in the biosphere and occupy diverse ecological niches. The morphological structure of the animals itself varies greatly between groups. The following common characteristics link animals into a meaningful classification group:
these are multicellular eukaryotic organisms
they do not have a cell wall
possess molecules of collagen, integrin, proteoglycans and adhesive glycoproteins in the extracellular matrix
possess specific intercellular connections such as fissure connections and desmosomes
there is the possibility of cell movement during ontogenetic development
have common features of zygote development - furrowing and blastula stage (possibly gastrula?)
they are heterotrophic, which conditions their roles in ecosystems (parasites, predators, detrivores)
possess a gastric cavity with one or two openings