QUOTE(Sponge Bob @ Jan 28 2004, 02:42 PM)
Tell us more about your kinky plans for the colony.
You say "creature" - I say "colony".
---------heallth warning: geekiness level goes off scale from here---------
It all depends on your view of what a "creature" is: one single sponge can be seen as a colony rather than a "creature", though there is some debate about that; and mainly because of that, they alone of all the animals are classified in the Parazoa (i.e., "almost animal") subkingdom.
The main reason for not saying that a sponge is a "creature" is that the cells are quite capable of surviving on their own, the sponge can be shaken apart, and some cells are totipotent (i.e. can transform themselves into all the other types of cells).
Which means that you can pass a sponge through a sieve and the different pieces will then form separate growing entities¹; in this sense, it's simpler to think of a single sponge cell as the "creature", and the sponge as a colony.
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¹I don't have to point out that this is much harder for entities that are unambiguously sa creature, e.g. a Sponge Bob.