>>67637504Cnidaria is the Phylum that contains all of the jellyfish, anemones, corals, siphonophores, etc. I'm not going to go into the taxonomic classification because, as always, it's a lot so unless you want me to talk about it I won't.
These animals are diploblastic, which describes animals which only contain two germ layers while they're developing as a blastula. For comparison, us humans and a lot of other animals are triploblastic, which means we have a third germ layer that later on becomes the body cavity, or coelom.
However, even though they lack a third tissue layer, they still have a third-psuedolayer. See, the first one, or the endoderm, becomes the animal's digestive tract, and the outer one or ectoderm becomes the animal's outer layer or skin. Cnidarians don't have the third one, mesoderm, but they still have a bundle of specialized tissue in between the two layers called the mesoglea. The reason this is considered a pseudolayer and not an actual one is because these cells aren't unique to the mesoglea, but instead come from the endo or ectoderm.
Cnidarians possess two distinct body configurations: the free-floating medusae and the sessile polyps. The former is what you usually picture when you think of a jellyfish and the latter are your corals and your anemone. Depending on what class of cnidarians the animal belongs to (cubozoa, hydrozoa, scyphozoa, or staurozoa
[not an exhaustive list]) will determine what configuration a sexually mature organism will be. In Scyphozoans adults usually tend to be medusae while in Hydrozoans a mature adult will be a polyp. I say usually because there's always so random ass animals in ass-middle-of-nowhere that will be an exception to this general rule.
There's a lot more to say, I mean several more paragraphs but I think this should be good enough for you now, tell me if you want to know more alright?
>>67637395Oh don't even get me started on that. So, since you brought it up I will assume you know a bit about their planula larva, right? The fact that their larvae are free-swimming, ciliated, bilaterally symmetrical organisms that later on go on to become radially symmetrical is actually insane. This little thing has served as a bridge between very primitive* animals and more complex* animals later down the line.
*
primitive and complex in this context don't mean less/more advanced, it just means that one of them is older than the other 