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In this entry of Wikibooks we find the claim

It is surprisingly difficult to find two texts that agree on a precise definition of an animal.

Is this true?

[I know nothing about biology, but I find this claim perplexing.]

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Definition of Animal

Animal is typically defined phylogenetically (just like any other monophyletic taxon; see here for intro to phylogeny). Any lineage part of the taxon Animalia (aka metazoa) is an animal. This is the definition given by wikipedia > Animal. You can find the animals here on the tree of eukaryotes.

Issue with the claim

It is surprisingly difficult to find two texts that agree on a precise definition of an animal.

This claim is pretty clumsy in my opinion. It sure can be hard to find a set of phenotypic traits that would group all animals together but it does not mean it is hard to define 'animal'.

IMO, what they meant is

It is surprisingly difficult to find a set of phenotypic traits the are shared by all animals and by none of the living organism that is are not animal.

This claim would make more sense. It is indeed true that it is hard to make such set of phenotypic traits. There are plenty of non-animals that are heterotrophic eukaryotes (such as fungi), there is at least one animal that is (at least partially) autotroph (Elysia chlorotica), not all animals have a nervous system, etc...

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    $\begingroup$ Thanks! +1. I tried to edit your answer by changing "triats" to "traits", but the software told me that edits must be at least 6 characters long. $\endgroup$ – Pierre-Yves Gaillard Jul 26 '17 at 19:40

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