Scientists have shown that animals as simple as the roundworm C. elegans, with a brain of just 302 neurons, need sleep to survive. Cassiopea has no brain to speak of — just a diffuse “net” of nerve cells distributed across their small, squishy bodies. These jellyfish barely even behave like animals.

Which organism is brainless?

The creature in question is called Stentor roeselii, a freshwater, single-celled organism that's shaped a bit like a trumpet. At one end of its relatively large body is a wide mouth-like opening, with tiny hairs called cilia surrounding it.

Which sea animal is brainless?

Some of the other animals that survive without brains include the sea star, sea cucumber, sea lily, sea urchin, sea anemone, sea squirt, sea sponge, coral, and Portuguese Man-O-War. A brain is basically what results when a large group of nerve cells called neurons form one large cluster.