Starfish and sea urchins are both echinoderms. There are very many types of echinoderms in the sea, in addition to the familiar starfish, sea urchins and sea cucumbers, there are also sea serpent tails (i.e., basket fish), sea lilies and so on. Echinoderms are a higher invertebrate with a five-radiation system.

The starfish is distributed in the Pacific Ocean region, living on the bottom of the sea, the bottom surface of the crawling life, its five-radial system is very obvious, the starfish represents a higher level than the sea anemone, the evolutionary development stage five tentacles outstretched, really like a beautiful five-pointed star.

The body of the starfish has a water pipe system, which is distributed all the way to the tentacles, which have tube feet that secrete mucus. Starfish rely on the suckers on the carpal feet to form a vacuum, it can be firmly attached to the substrate. The starfish's wrists have a very strong force and can pull apart the tightly closed shells of bivalves. It is also thought that the starfish can also secrete chemicals that anesthetize the bivalve's closed-shell muscles and then turn its stomach out to wrap the shelled animals and mollusks up and eat them.

The starfish is a carnivorous species that mainly eats mollusks as gastropods or bivalves. The starfish is a peculiar eater, relying on its body wall to contract, spewing out the cardinal stomach, which flips out of the body, wraps around the food to digest it, and then retracts to deliver the food to the mouth.

Because the starfish feeds mainly on bivalves, it is extremely harmful to farmed scallops and others, and is considered an enemy of the farming industry.

The sea urchin, with a rounded body shape, is covered with spines. Observing empty sea urchins, which are calcareous urchins, you can see that there are also obvious traces of five radials on the scalloped round urchins.

The sea urchin is distributed in the Indian Ocean and the western Pacific Ocean, is a warm-water species, preferring to inhabit in the intertidal zone below the sea area between the reefs, or the harder muddy and sandy shallow sea area, cut holes drilling holes, submerged seabed life.

Sea urchins are covered with spines, and some species have poison glands on their spines. Sea urchin movement can only be adjusted slowly with the spines, so it has a small range of activities, only foraging near the burrow.

Sea urchins have different "diets" depending on the species. Some species are "vegetarians," feeding on algae; others are omnivorous, feeding on everything from protozoa to microorganisms; others are carrion-eaters, feeding on the carcasses of other animals; and still others are carnivorous, feeding on attached organisms on rocks or coral reefs, and on small, slow-crawling animals. Some sea urchins are also hostile to farming.

Echinoderms are also of great economic value. In addition to the valuable sea cucumber, sea urchins are also edible. Every summer and fall, when the sea urchin's gonads are ripe, it is the golden season for harvesting sea urchins, and eating sea urchins means eating their gonads (sea urchin yellow). Processed sea urchins are valuable food for export. Many countries have put them on the "national feast".