“Animals are eukaryotic and multicellular,[7] which separates them from bacteria and
most protists. They are heterotrophic,[8] generally digesting food in an internal chamber,
which separates them from plants and algae.[9] They are also distinguished from plants,
algae, and fungi by lacking rigid cell walls.[10] All animals are motile,[11] if only at
certain life stages. In most animals, embryos pass through a blastula stage,[12] 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 tissues, which send and process signals.
Typically, there is also an internal digestive chamber, with one or two openings.[13]
Animals with this sort of organization are called metazoans, or eumetazoans when the
former is used for animals in general.[14]
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix
composed of collagen and elastic glycoproteins.[15] This may be calcified to form
structures like shells, bones, and spicules.[16] During development, it forms a relatively
flexible framework[17] 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.[13]
Also, unique to animal cells are the following intercellular junctions: tight junctions, gap
junctions, and desmosomes.[18]”
Citation: From Wekipediaa