oldest multi organ animal

the-indifferent-universe  asked:

Jellies! I went to an aquarium today and I am completely fascinated by them! So, what's the deal with jellies? What do their gelatinous bodies consist of, structure wise? Also, are they made up of jelly or jam? Strawberry...or grape?

Actually, marmalade.

No but really, weirdly enough, jellyfish are actually made up primarily of water—depending on the species, between 95% and 98% of a jellyfish’s body mass is water.

What we’d call the “jelly” of a jellyfish is actually a thick, elastic substance called mesoglea, which is sandwiched on either side by two layers: the outer epidermis, and the inner gastrodermis. The mesoglea is mostly water but also composed of fibrous proteins, and it essentially serves as a skeleton-substitute, keeping the jellyfish’s body together.  It needs water to support its weight, though, which is why when you take a jellyfish out of water, suddenly it’s no longer a mysterious and elegant creature; it’s just a blob. If a jellyfish washes up on a beach, it will collapse and pretty much almost disappear as all the water inside of it evaporates.

So that’s the deal with their jelly, but jellyfish are actually incredibly cool so here are a few more tidbits:

They belong to the phylum Cnidaria, and like all members of that phylum, they’re radially symmetrical. This means that their body parts radiate out from a central axis, which runs the length of their body from the top to the end of their tentacles. If you cut a jellyfish in half at any place along that axis, you’d get two symmetrical halves. This is pretty useful, because it allows them to respond to food or danger from all directions.

Even weirder is how they even know there’s food or danger to respond to. Jellyfish have no brains, no blood, no heart, and no central nervous system—but they do have a very elementary network of nerves located in the epidermis. Called the “nerve net”, it can detect a variety of stimuli including the touch of other animals, to help with catching prey. Some jellyfish also have ocelli, which are rudimentary eyes—they’re light-sensitive organs that don’t form images, but they detect light and can help the jellyfish tell up from down, so they can orient themselves.

The box jellyfish in particular has 24 eyes, two of which can see colour, and it’s one of the very few creatures that has a 360 degree view of its environment. I was TERRIFIED of them as a child, and it looks like that was for good reason. (Also, box jellyfish are one of the most venomous creatures on the planet, so that might have had something to do with it.)

Their blobby bodies, weird eyes and terrifying tentacles must work pretty well for them, because jellyfish have roamed the oceans for at least 500 million years, possibly even longer–and they’re actually the oldest multi-organ animal!