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Jelly, for one thing. Ctenophores aren't a great deal more than a load of jelly held between two layers of skin, and the jelly itself isn't much more than water. They have no brain, which you probably suspected, instead making use of a nerve net just as jellyfish do. They have a mouth where the food goes and an internal cavity which serves as a stomach. Food here is various forms of plankton. A tiny 1 mm long Comb Jelly will eat microscopic organisms and maybe eggs. Larger ones could manage adult Copepods and maybe even small Krill, while some eat Salps, jellyfish or other Comb Jellies. Of course, they have to catch them first!
At this point it's probably a good idea to start off with the classic Comb Jelly. It will be round or egg shaped and live some distance from the bottom of the sea. All Comb Jellies have jelly, very nearly all of them also have 8 rows of combs. These are cilia, or hairs, which can reach as much as 2 mm long in some species. This is exceptionally long for cilia because Comb Jellies are the biggest animals to use them for locomotion. They beat them in a Mexican wave style so that they can slowly and rather weakly swim around with their mouth facing forward. There are thousands of these hairs and they can be seen to scatter light, creating lovely rainbow colours. A lot of Comb Jellies are bioluminescent, but it's usually blue or green. They can't actually sing a rainbow! (Has that song always been so emotional and cry baby? I thought it was supposed to be fun!)
To actually grab hold of prey, Comb Jellies use a pair of retractable tentacles covered in colloblasts. These are special cells with a kind of sticky glue used to entangle food; they don't sting like Cnidarians do. The tentacles often also have a fringe of little tentacles branching off the main one, these are delightfully named "tentilla".
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It's all much more interesting than I'd ever thought, and I can't wait to explore some of these oddities!
When it comes to reproduction, Comb Jellies are almost all hermaphrodite. Sperm and eggs are simply released into the water and what will be, will be. They do this when there is enough food but they'll stop once they start getting too hungry. This is a great idea, but they can go one step further and actually shrink in size if food continues to be scarce! If a phytoplankton bloom causes a huge increase in Salps, Copepods or Krill, Ctenophores are all set to start eating, growing and reproducing on a potentially massive scale. Very clever! Particularly for something that could so easily be described as a bag of water.