Sea stars, also known as starfish, are echinoderms belonging to the class Asteroidea. The names "sea star" and "starfish" are sometimes differentiated, with "starfish" used in a broader sense to include the closely related brittle stars, which make up the class Ophiuroidea.

Sea stars exhibit a superficially radial symmetry. They typically have five "arms" which radiate from an indistinct disk (pentaradial symmetry). However, the evolutionary ancestors of echinoderms are believed to have had bilateral symmetry. Sea stars do exhibit some superficial remnant of this body structure, evident in their larval pluteus forms.

Sea stars do not rely on a jointed, movable skeleton for support and locomotion (although they are protected by their skeleton), but instead possess a hydraulic water vascular system that aids in locomotion. The water vascular system has many projections called tube feet on the ventral face of the sea star's arms which function in locomotion and aid with feeding. Sea stars usually hunt for shelled animals such as oysters and clams. They have two stomachs. One stomach is used for digestion, and the other stomach can be extended outward to engulf and digest prey. This feature allows the sea star to hunt prey that is much larger than its mouth would otherwise allow. Sea stars are able to regenerate lost arms. A new sea star may be regenerated from a single arm attached to a portion of the central disk.

Brittle stars, or ophiuroids, are echinoderms, closely related to sea stars. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to 60 centimeters (2 feet) in length on the largest specimens. They are also known as serpent stars.

Ophiuroidea contains two large clades, Ophiurida (brittle stars) and Euryalida (basket stars). Many of the ophiuroids are rarely encountered in the relatively shallow depths normally visited by humans, but they are a diverse group.

There are some 1,500 species of brittle stars living today, and they are largely found in deep waters more than 500 metres (1,650 feet) down.

The Crown-of-Thorns Starfish (Acanthaster planci) is a large nocturnal starfish that preys upon coral polyps. The crown-of-thorns receives it name from venomous thorn-like spines that cover its body.

The Crown-of-Thorns is endemic to tropical coral reefs in the Red Sea, the Indian Ocean, and the Pacific Ocean. Solitary animals, they feed alone and maintain constant distance between themselves and other members of their species.

The Crown-of-Thorns is the second largest starfish in the world. Only the Giant Sunstar is larger.

Protoreaster nodosus is a species of starfish found in the warm, shallow waters of Indo-Pacific region. Also known as "horned" or "Chocolate Chip"Shedd Aquarium sea stars, they possess rows of spines or "horns"; black conical points arranged in a single row, radially on the dorsal side, which may erode and becoming blunt. On the ventral side, tube feet, purple in color, are arranged in doube rows on each arms. Most horned sea stars found are roughly rigid five-pointed star-shape with tapering arms to the end, although there are anomalies like four or six-armed specimens; they may grow up to 30 centimetres in diameter. Usually colored in shades of red or brown, horned sea stars prefer sheltered, sandy or slightly muddy bottoms more than hard substrata such as coral reef, and are frequently sighted conspicuously between the leaves of seagrasses on sea grass meadows or on blank stretch of coral sand. In shallow water, this species can be seen intertidally, occasionally exposed to the low tide. Sometimes, many individuals of this species can be seen gathering on soft bottom with reason not very well known, probably to increase the chance of fertilization when spawning or simply a suitable feeding ground.

Horned sea stars seem to be opportunistic carnivores; adults are known to predate on most sessile life forms including hard corals and sponges in aquarium; An individual of horned sea star is also has been observed eating a sea urchin in their natural habitat Gremli MS, Newman HE, Insight Guides Underwater: Marine Life In the South China Sea, page 159. APA Publications GmbH & Co. Verlag KG., 1993. In public aquariums, these stars are sometimes target-fed with chopped squids, clams or shrimps, and are popular animals for touch pools as it is moderately large in number in some undisturbed areas. Horned sea stars need perfect water condition, suitable diet and proper acclimatization to thrive - thus, these sea stars did not fare very well caused by several factors mentioned before in many home aquariums. In reef aquariums, they are regarded as undesirable as they have destructive tendencies of toppling decors and digesting patches of corals and sessile organisms; they are reasonably safe if kept in a fish-only aquarium.

In many tropical Asian and Pacific countries, horned sea stars are collected for sea shell trade for their ornate skeletons. In some areas overharvesting is an issue and reduces greatly the population of the echinoderms. Related to this species, the Atlantic Oreaster reticulatus or commonly known as Bahama Sea Star, is also a previously-abundant species reduced in number due to the continuous harvesting by the industry and tourists alike.

As with other tropical echinoderms, commensal animals like shrimps (of genus Periclimenes), tiny brittle stars and even juvenile filefish can be found on the surfaces of a horned sea star. This may be attributed to its protective nature, as there are few predators would dare to eat this echinoderm.

Crinoids, also known as sea lilies or feather-stars, are marine animals that make up the class Crinoidea of the echinoderms (phylum Echinodermata). They live both in shallow water and in depths as great as 6,000 meters. Crinoids are characterized by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. Although the basic echinoderm pattern of five-fold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers in the past. Some thick limestone beds dating to the mid- to late-Paleozoic are entirely made up of disarticulated crinoid fragments.

Sea urchins are small, globular, spiny sea creatures, composing most of class Echinoidea. They are found in oceans all over the world. Their shell, or "test", is round and spiny, typically from 3 to 10 cm across. Common colors include black and dull shades of green, olive, brown, purple, and red. They move slowly, feeding mostly on algae. Sea otters, wolf eels, and other predators feed on urchins. Sea urchins are harvested and served as a delicacy.

Sea urchins are members of the phylum Echinodermata, which also includes starfish, sea cucumbers, brittle stars, and crinoids. Like other echinoderms they have fivefold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The pentamerous symmetry is not obvious at a casual glance but is easily seen in the dried shell or test of the urchin.

Together with sea cucumbers (Holothuroidea), they make up the subphylum Echinozoa, which is defined by primarily having a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved different shapes. Although many sea cucumbers have branched tentacles surrounding the oral opening, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars (starfish) and brittle stars.

The sea cucumber (also known as trepang, bêche-de-mer, balate, or ambiguously, sea slug) is an echinoderm of the class Holothuroidea, with an elongated body and leathery skin, which is found on the sea floor worldwide. It is also named because of its cucumber-like shape. The body contains a single, branched gonad.

Like all echinoderms, sea cucumbers have an endoskeleton just below the skin, calcified structures that are usually reduced to isolated microscopic ossicles (or sclerietes) joined by connective tissue. These can sometimes be enlarged to flattened plates, forming an armour. In pelagic species (Order Elasipodida, family Pelagothuriidae), the skeleton is often absentCambrian holothurians ? ? The early fossil record and evolution of Holothuroidea.