I know this sounds like a really stupid question but just bear me out.
I was just watching a TV program about jellyfish and they really puzzled me. They seem to be nothing more than a sac and a load of tentacles - no brain, or indeed any other internal organs.
Should they even count as animals at all? In many ways they seem to act more like carnivorous plants. Is this a more accurate description of them or are they something completely different?
Cnidarians (which include true jellyfish, Hydra, sea anemones and Portugese man-o-war.) are actually more complex than you might think. They have light receptors, balance organs, gonads, stomachs, gastric pouches, a gastrovascular system and a network of musculature.
They are very basic metazoans (multicellular animals). But the phylum-wide absence of general plant characteristics (cell walls, photosynthesis, plasmodesmata) makes it inaccurate to consider them plants. Sponges are also rather primitive mutlicelled organisms, even simpler than jellyfish, but they are still considered animals.
In other words, just because an organism is relatively simple looking doesn’t make it a plant.
I have seen discussions of the man-o-war that suggest that it behaves more like a group of several functionally distinct cell colonies working together than a single animal. But it still ain’t a plant. 
They don’t work, those no-good drifters. Any more than some of the other mentioned organisms that just sit around and sponge off the undersea social order.
I think the question is: How do they function? I’ve wondered this myself. They don’t seem to have any muscles. How do they contract? What causes the tentacles to dart toward the digestive system?
As I said above, cnidarians do have muscles. These muscles are ennervated by two separate nerve networks. It are these muscles that control the locomation of the jellyfish. In fact, one cluster of nerves acts as a pacemaker, ensuring that the bell of the jellyfish contracts rhythmically.
My WAG is that despite the lack of a central nervous system, the sensation of grasped prey is enough to trigger a reflex arc that causes the movement of the prey towards the mouth of thejellyfish.
I can’t fully answer the question of how jellyfish function, muscle-wise, but I think it’s interesting that a lot of the fluids that would be specialized in a more complex organism are just seawater in a jellyfish. I mean, they kind of have seawater for blood - there’s no point in evolving an independent circulatory system if sloshing brine will do the job for you.
The whole subject is fascinating to me, and I confess ignorance. In any case, from my layman’s perspective, a jellyfish seems in some ways like a giant protist. Jellyfish (or some of them) are colony organisms, sort of a coalition of cells working together. So a lot specialized tissue that exists in fish or wasps or whatever doesn’t exist in jellyfish. I suppose it raises questions about what is an organism and what is a community. The answer, I think, is that an organism springs from a single fertilized germ cell, while a community springs from many.