Equinodermos

Equinodermos, or echinoderms, are a phylum of exclusively marine deuterostome animals characterized by pentaradial symmetry as adults, an internal calcareous endoskeleton, a water vascular system with tube feet, and a nerve net. They inhabit oceans worldwide, from shallow coastal zones to deep-sea trenches, often on hard substrates or among sediments. There are about 7,000 described species, grouped into five main classes: Crinoidea (sea lilies and feather stars), Asteroidea (starfishes), Ophiuroidea (brittle stars and basket stars), Echinoidea (sea urchins and sand dollars), and Holothuroidea (sea cucumbers).

Anatomy includes a calcitic endoskeleton made of ossicles, a characteristic water vascular system that powers locomotion and feeding through tube feet, and a wide, nerve net rather than a centralized brain. Many species bear pedicellariae, small pincer-like appendages, and some possess mutable connective tissue that can rapidly stiffen or soften their bodies. Adults commonly display pentaradial symmetry, while larvae are typically bilaterally symmetric.

They are exclusively marine and occupy a variety of feeding modes, including predation, grazing, filter feeding, and detritivory. Reproduction is usually sexual with separate sexes; many species release gametes into the water column for external fertilization. Larval development often includes free-swimming, bilateral stages such as the bipinnaria and echinopluteus forms, before metamorphosis into the adult radial body plan.

Taxonomically, echinoderms are part of the Ambulacraria and are more closely related to chordates than to most invertebrates. The fossil record reaches the Cambrian period, indicating a long and ancient lineage. Ongoing threats include habitat loss, pollution, and ocean acidification, which can affect calcified skeletons and reef-associated echinoderms.