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|image=Vulpes vulpes.jpg

|image=Vulpes vulpes.jpg

|kingdom=Animalia}}

|kingdom=Animalia}}

An '''animal''' is any creature which is a member of the [[Animalia]] kingdom. Animals must eat [[plant]]s or other organisms to survive. The plants are called "producers" and the animals are "consumers". Scientists call consumers [[heterotroph]]s. Animals have complex [[eukaryotic cell]]s, unlike bacteria which have much simpler [[prokaryote|prokaryotic]] cells. Animal cells do not have [[cell wall]]s, only structural [[protein]]s in the [[cell membrane]]; this allows the animal cell a certain amount of flexibility.

An '''animal''' is any creature which is a member of the [[Animalia]] kingdom. Animals must eat [[plant]]s or other organisms to survive. The plants are called "producers" and the animals are "consumers". Animals have complex [[eukaryotic cell]]s, unlike bacteria which have much simpler [[prokaryote|prokaryotic]] cells. Animal cells do not have [[cell wall]]s, only structural [[protein]]s in the [[cell membrane]]; this allows the animal cell a degree of flexibility.

The simplest of all animals are [[sponge]]s, which do not move.  Sponges have different cells for different tasks, but these cells are not organized further into [[tissue]]s or [[organ]]s.  Sponges also do not exhibit [[symmetry]], unlike all other animals.

The simplest of all animals are [[sponge]]s, which do not move.  Sponges have different cells for different tasks, but these cells are not organized further into [[tissue]]s or [[organ]]s.  Sponges also do not exhibit [[symmetry]], unlike all other animals.

The next simplest animals comprise a single species, which fall somewhere between sponges and all other animals.  This species, ''Trichoplax adhaerans'', is very small, yet has cells organized into different kinds of tissues, though it does not have organs.  This is an improvement on the sponge body plan.

The next simplest animals comprise a single species, which fall somewhere between sponges and all other animals.  This species, ''Trichoplax adhaerans'', is very small, yet has cells organized into different tissues, though it does not have organs.   

True Animals are grouped into a sub-kingdom called Eumetazoa.  All True Animals have tissues and organs, except for one type called Myxozoa. These have regressed to a single-celled form.

True Animals are grouped into a sub-kingdom called Eumetazoa.  All True Animals have tissues and organs, except for one type called Myxozoa. These have regressed to a single-celled form.