ekinodermata

Echinodermata, sometimes spelled ekinodermata in older or less common transliterations, is a phylum of marine invertebrates that includes starfish, sea urchins, sand dollars, brittle stars, crinoids, and sea cucumbers. Echinoderms are deuterostomes and are characterized by pentaradial symmetry as adults, a calcareous endoskeleton, and a water vascular system with tube feet used for locomotion, feeding, and respiration.

Most have a body plan organized around a central disk with five or more radiating arms (in crinoids the arms can be branched). The endoskeleton consists of ossicles made of calcium carbonate, and the body is covered by a thin epidermis. They lack traditional excretory, circulatory, and respiratory organs; gas exchange and waste diffusion occur largely across respiratory surfaces and tube feet. A distinctive feature is mutable connective tissue that can rapidly change stiffness, aiding movement and posture.

Echinoderms inhabit marine environments from shallow coastal waters to the deep sea and often play key ecological roles as predators, grazers, or detritivores. Reproduction is typically sexual, with separate sexes and external fertilization; many species also exhibit regenerative abilities, and some reproduce asexually by fragmentation.

Development usually includes bilateral larval stages that metamorphose into radial adults. The fossil record extends to the Cambrian, and living echinoderms are grouped into five main classes: Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Crinoidea (feather stars and sea lilies), and Holothuroidea (sea cucumbers).