Jellyfish contain neurons that send messages throughout their bodies, but no brains. Jellyfish lack lungs and hearts. Their delicate skin absorbs oxygen.
Sea urchins lack a brain. Instead, a nerve ring surrounds their mouth. Sea urchins are 1 to 4 inches long. The red sea urchin can grow up to 7 inches.
Starfish may digest food outside their body despite lacking blood or a brain. Not even that makes these brainless beasts interesting. Starfish can renew their arms, but it takes months or years.
No sea sponge has a brain or other internal organs. These marine invertebrates feed on algae, bacteria, and other microorganisms that float by.
Sea lilies aren't actually plants. Invertebrates without brains. They reside on the sea floor but can float away. 6ft sea lilies feed on ocean garbage.
In spite of their lack of brains, corals nevertheless possess a nervous system. This structure extends from the animal's mouth to its tentacles. They average about 2 centimetres per year in growth.
Neurologically, they can react and experience things. Clams live in saltwater and freshwater. The largest of 15,000 clam species are giant clams. 4 foot-long, 100-year-old giant clams exist.
Oysters have a nervous system, organs, and heart, but no brain. Oysters can filter 1.3 gallons per hour. Oysters can be kept 20 years.
They're connected to corals and jellyfish. Sea anemones lack a brain but have a nerve system, albeit a simple one. Their nervous system allows them respond to stimuli.