Starfish are Echinoderms, meaning ‘spiny skinned animals’ of the class Asteroidea, in which there are around 1500 species. They are not fish and should really be called sea stars. Some species of starfish always have five rays, but some always have more – sunflower stars can have over 20 - and some species are variable.
Starfish aren’t edible to many creatures – their spiny skin and awkward shape makes them distasteful to most. They also have tiny structures like pincers, called pedicellariae, that are probably used for defense as well as capturing small particles of food. They are found all over the world, and at all depths of the sea, though they tend to favor rocky shallows. 
Starfish eat almost anything that cannot easily escape, but mainly bivalves. They eat these by using their tube feet as suction cups to prise a small gap in the bivalve’s shell, and then reverting their own stomach through their mouth opening and extending it into the shell, using their digestive juices to liquefy the creature inside and then sucking it in. Starfish help the intertidal ecosystem stay in balance by eating mussels, which would otherwise push other organisms out.  The crown-of-thorns starfish (Acanthaster planci), which feeds on coral, is decimating coral reefs off the Philippines and Australia.
A starfish doesn’t have a front end; any side of a starfish is as likely to be the front as any other side. They also don’t have ears or image-forming eyes, though they do have eyespots on the end of each arm containing a red pigment that changes chemically in the presence of light and are thought to influence the starfish's behavior. 
Starfish don’t have a central brain. A radial nerve runs down each ray, and connects with the other radial nerves through ganglia that form a ring in the center of the starfish. Because the starfish’s mouth is in the center of the underneath of its body, this is called the circumoral nerve ring. It does not control the rays though, merely facilitates communication between them. Any radial nerve can take control of the body of the starfish by signaling the others to co-ordinate movement in the direction of the ray it runs down. However, as well as locating food and mates, starfish can learn to associate different textures of seabed or levels of light with food, and as well as distinguishing between odors can learn to ignore those not associated with food even if they smell like food.
Like a lot of other marine creatures, a starfish’s primary sense is chemoreception. Leading rays tend to raise their tips and wave their tube feet around. They use their tube feet to sense the concentration of chemicals, comparing the intensity of the information between rays and moving accordingly to reach the source of the odor, so it would be true to say that they have multiple noses in their feet. Because starfish are too primitive to have consciousness, their sleeping and waking cycles can only be determined by different firing rates of nerves in response to changes in light intensity. They feed in response to odours from prey. 
Hundreds of tube feet cover the underside of a starfish, and have multiple uses. As well as chemoreception, and respiration, the starfish uses them for walking, for gripping prey, and for attaching to rocks. Each tube foot has a bulb at the top called the ampulla. When the ampulla contracts, it squeezes water down the tube foot, which extends. When it relaxes, it sucks water back up, which retracts the tube. Each tube foot lifts up and swings forward, plants itself on the ground and pushes back, rather like a leg. The tube feet and the ampullae are connected by the starfish’s hydraulic water vascular system, which takes in water through a body called a madreporite, which filters out particles, and circulates it throughout the starfish’s body. The tip of each foot can form a suction cup, which the starfish can use to grip enough to walk up vertical surfaces. The tube feet move in waves, and enable starfish to move at speeds of between 30 and 120 centimeters a minute, depending on species. 
Starfish breathe through their feet. The starfish’s tube feet are made of very thin tissue, so gases can move through them easily. Their tube feet and papulae (the tiny pimples all over the body surface) transport oxygen from seawater to the tissues and pass carbon dioxide from the tissues to the seawater. The papulae have tiny cilia (hair-like structures) on the outside and the inside, which waft seawater over the tissues. 
Most starfish species have two sexes, but you can’t tell that from the outside – you have to catch them spawning. They have ten gonads, two in each arm, that look like a bunch of grapes. When they are full of sperm or eggs, they almost fill the arms. Each gonad has a duct at the base of the arm, usually opening on the oral side. In a few species the eggs are fertilized and develop within the female (especially in colder water) but they are mainly fertilized externally. The male releases his sperm into the water if he detects the eggs of a female in the water, and the female releases her eggs if she detects sperm.
Starfish gather in groups when they are ready to spawn, to increase their chances of fertilization. They probably use environmental signals such as day length to know the right time of year. Females can release up to 2.5 million eggs at a time. The larvae swim freely, propelled through the water by the cilia covering them. Eventually they grow arms and a sucker, sink to the bottom and stick to a rock, after which they metamorphose into adult starfish. Larvae, unlike adults, are bilateral.
Most starfish can regenerate a lost arm, but most of the central disc has to remain for this to happen. Single arms will usually die, unless they are from a species called Linckia, which can regenerate a new individual from one arm, because they keep so much of the vital organs in each arm. This species is sometimes called the comet star because you can see an animal with one large arm, a tiny body and four tiny arms, looking like a comet. 
Starfish eat almost anything that cannot easily escape, but mainly bivalves. They eat these by using their tube feet as suction cups to prise a small gap in the bivalve’s shell, and then reverting their own stomach through their mouth opening and extending it into the shell, using their digestive juices to liquefy the creature inside and then wafting the liquefied juices into its mouth with its tube feet. This enables a starfish to hunt much larger prey than it could actually fit into its mouth. Starfish help the intertidal ecosystem stay in balance by eating mussels, which would otherwise push other organisms out.  The crown-of-thorns starfish (Acanthaster planci), which feeds on coral, is decimating coral reefs off the Philippines and Australia.
Starfish have a mouth in the middle of their underside (the ‘oral’ side) and an anus on their upper surface (the ‘aboral’ side). The mouth connects to the first of two stomachs – the cardiac stomach – which connects to the pyloric stomach. This connects to the anus and also to a pyloric cecum, which extends into each ray. The cardiac stomach and the pyloric ceca produce digestive juices. Digested material is absorbed into the body through the pyloric ceca.
Use in scientific research
In 1882 Russian zoologist Ilya Metchnikov discovered the concept of immunity – how human beings survive in a world filled with dangerous microorganisms - when he poked a starfish with a thorn and watched the defensive inflammatory actions of its body as it used cells to engulf and destroy foreign pathogens. He won a Nobel Prize for this work in 1908.
Starfish in popular culture
Madeleine L’Engle used starfish regeneration as an image of the ethics of studying the abuse of the natural world, on a fictional island called Gaea, in her 1965 novel The Arm of the Starfish. 
Starfish have been used as a metaphor by countless good causes who picked up a story from Loren Eiseley’s book The Star Thrower, in which thousands of starfish are washed ashore and a child starts throwing them back in the water so they don’t die. Her mother tells her it will make no difference, and the child looks at the starfish it is holding and says, ‘It will make a difference to this one.’ 
The best friend of beloved children’s TV cartoon character SpongeBob Squarepants, with whom he often holds hands, is a pink starfish named Patrick. The duo have become popular among gay men, and as such have been attacked by the prominent Christian evangelical organization Focus on the Family as part of a conspiracy trying to impose a pro-homosexual agenda on children. 
A recent book called The Starfish and the Spider uses starfish as a metaphor for decentralized organizations. Traditional organizations are like spiders – cut off the head and they die. Especially since the arrival of the internet, the power of starfish organizations like Alcoholics Anonymous, P2P groups, eBay or Al Quaeda – individual groups linked loosely by a common ideology and ease of communication – has boomed. It’s not a totally new concept, and has much in line with earlier movements such as that for women’s rights or the abolition of slavery, but some large companies like Toyota are seizing on these principles.
Project Starfish was a series of experiments carried out by the US in 1962, which were condemned by the Queen’s Astronomer, Sir Martin Ryle. The US detonated several nuclear explosions in the ionosphere that seriously disturbed the lower Van Allen Belt, and thus disturbed the earth’s magnetic field and released ionizing radiation into the atmosphere. The USSR soon followed suit, and the electron fluxes in the lower Van Allen belt have yet to return to their normal state. They could take several hundred years to recover. 
Sigmund Freud loved his trip to Blackpool, and wrote in chapter 7 of The Interpretation of Dreams about a dream based on a memory of his visit, when while he was paddling on the beach a little girl asked if he had found a starfish and if it was still alive. Freud replied ‘Yes, he is alive’ and was then embarrassed at his linguistic mistake and repeated the sentence correctly.
- The Arm of the Starfish, L’Engle
- Encyclopaedia Britannica
- The Interpretation of Dreams, Sigmund Freud