DavidPT40:
I have heard often that jellyfish are a single animal but a group of organisms living together in a community. Is this true?
Those would be the siphonophores, a group of jellies bearing more of a "colonial cellular" habit more than a tissue-scale one. The classical known (but atypical) member is the Portuguese Man 'O War. Structurally, the organism is an assemblage of various specialized polyps (stinging, digestive, reproductive, floating).
I've never really agreed with this statement, because wouldn't any animal with specialized cells be a group of organisms living together?
Yes. All animals are however, multicellular. The term is generally equated with the metazoan lines, and usually the parazoans (sponges and placozoans) as well.
There's a tier of sorts regarding multicellularity in living forms.
1. Unicellular
2. Unicellular living in loose assemblages
3. Simple Colonial (large assemblages, little specialization)
4. Complex Colonial (few specialized cell types)
5. Simple Multicellular (specialized cells, semi-tissues)
6. True Multicellular (true tissues)
The Man 'O War is still multicellular, not colonial. The polyps themselves are complex with basic tissues. As are all hydroids.
