Echinodermata is a phylum of exclusively marine, bottom-dwelling (largely benthic) invertebrates that are among the most morphologically distinctive and evolutionarily significant animal groups. Approximately 7,000 living species are known, distributed across all ocean depths from the intertidal zone to hadal depths exceeding 10,000 metres. Despite their radically different adult body plan, echinoderms are deuterostomes — they share embryological development features (blastopore becoming the anus rather than the mouth, enterocoelic coelom formation, and radial/indeterminate cleavage) with chordates, making echinoderms the invertebrate phylum most closely related to the vertebrates. This evolutionary relationship, rather counter-intuitive given the starfish's apparent distance from any vertebrate, is strongly supported by molecular phylogenetic evidence and explains why echinoderms are studied as outgroup models in evolutionary developmental biology research on chordate body plan origins.

Exclusively marine: no freshwater or terrestrial echinoderms exist. Triploblastic eucoelomate: three embryonic germ layers (ectoderm, mesoderm, endoderm) and a true coelom (enterocoel, formed from outpockets of the archenteron/primitive gut during development). Endoskeleton: calcite (calcium carbonate) ossicles embedded in the dermis, often bearing spines that project through the epidermis (explaining the phylum name "spiny skin"). Water vascular system: hydraulic system for locomotion and feeding, unique to echinoderms. Pentaradial symmetry in adults: 5-fold (or multiples of 5) radial symmetry secondary derived from a bilateral ancestral condition. No head or distinct brain: nervous system is decentralised, with a nerve ring around the oesophagus and radial nerves extending into each arm/section. Remarkable regeneration ability: can regenerate lost arms and in some species the entire body from a single arm plus part of the central disc. Separate sexes (mostly): most echinoderms have separate male and female individuals with external fertilisation.

The transformation from bilaterally symmetrical larvae to pentaradially symmetrical adults is one of the most remarkable developmental transitions in the animal kingdom, representing a fundamental reorganisation of the entire body plan during metamorphosis. Starfish (Asteroidea): fertilised egg → Bipinnaria larva (elongated, bilaterally symmetrical, ciliated, free-swimming) → Brachiolaria larva (larger bipinnaria with additional adhesive organ for settlement) → settles and metamorphoses into juvenile starfish with pentaradial symmetry. Sea urchin (Echinoidea): egg → Pluteus larva (bilaterally symmetrical, transparent, with skeletal rods giving a tripod-like appearance) → metamorphosis into juvenile urchin. Sea cucumber (Holothuroidea): egg → Auricularia larva → Doliolaria larva → metamorphosis. Sea lily (Crinoidea): egg → Doliolaria larva → Cystidean larva → adult sea lily. The transition from bilateral to radial symmetry during metamorphosis likely reflects the evolutionary history of the phylum — echinoderms evolved from bilaterally symmetrical ancestors (as evidenced by their bilaterally symmetrical larvae and their deuterostome relationships), with the pentaradial symmetry of the adult representing a derived, secondary condition that evolved in association with their sessile or slow-moving bottom-dwelling lifestyle.

Five living classes are recognised: Asteroidea (sea stars/starfish): flat, star-shaped with 5 or more arms; tube feet with suckers; carnivores that can evert their stomach to digest prey externally. Examples: Asterias, Pentaceros. Ophiuroidea (brittle stars): similar to starfish but with distinct central disc and long, snake-like arms that move by lateral bending (unlike sea stars' tube feet locomotion); largely detrivores/filter feeders. Examples: Ophiothrix. Echinoidea (sea urchins and sand dollars): body enclosed in rigid test (fused ossicle plates); movable spines; tube feet for locomotion; well-developed Aristotle's lantern (chewing apparatus) in sea urchins. Examples: Echinus, Strongylocentrotus. Holothuroidea (sea cucumbers): elongated body along oral-aboral axis; no arms; ossicles small and scattered (body feels leathery); can eviscerate when threatened. Examples: Holothuria, Cucumaria. Crinoidea (sea lilies and feather stars): considered the most primitive living class; stalked (sea lilies) or free-moving (feather stars); arms are feathery and used for filter feeding. Examples: Antedon, Ptilocrinus.