The word "animal" comes from the Latin word animal (meaning with soul, from anima, soul). The biological definition of the word refers to all members of the Kingdom Animalia including humans.[2]
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and are multicellular[3] (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic,[4] generally digesting food in an internal chamber, which separates them from plants and algae (some sponges are capable of photosynthesis and nitrogen fixation though).[5] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[6] All animals are motile,[7] if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.
Structure
With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissue, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.
"We submit to the majority because we have to. But we are not compelled to call our attitude of subjection a posture of respect." Ambrose Bierce
An animal is any living, breathing creature.
The word "animal" comes from the Latin word animal (meaning with soul, from anima, soul). The biological definition of the word refers to all members of the Kingdom Animalia including humans.[2]
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and are multicellular[3] (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic,[4] generally digesting food in an internal chamber, which separates them from plants and algae (some sponges are capable of photosynthesis and nitrogen fixation though).[5] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[6] All animals are motile,[7] if only at certain life stages. In most animals, embryos pass through a blastula stage, which is a characteristic exclusive to animals.
Structure
With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissue, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form structures like shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.
"We submit to the majority because we have to. But we are not compelled to call our attitude of subjection a posture of respect." Ambrose Bierce