Echinodermi

Echinodermi, or echinoderms, constitute a phylum of exclusively marine invertebrates characterized by a calcareous endoskeleton, a water vascular system, and a pentaradial body plan in most adults. The group includes sea stars (Asteroidea), sea urchins and sand dollars (Echinoidea), brittle stars (Ophiuroidea), feather stars and sea lilies (Crinoidea), and sea cucumbers (Holothuroidea). About 7,000 described species inhabit oceans worldwide, from shallow coastal habitats to the deep sea.

A hallmark of echinoderms is their radial symmetry in adults, typically fivefold, arising from bilateral larvae. The endoskeleton consists of calcareous ossicles embedded in connective tissue. The water vascular system, derived from coelomic spaces, powers locomotion and feeding through tube feet. Respiration occurs via thin-walled structures or dermal gills (papullae) in many species. The nervous system is a nerve ring around the mouth with radial nerves, and there is no centralized brain.

Reproduction is mainly sexual, with separate sexes and external fertilization in many species. Development involves free-swimming larvae, such as bipinnaria or pluteus stages, followed by metamorphosis into the adult form. Asexual reproduction by fragmentation and regeneration occurs in some starfishes and other groups. Ecologically, echinoderms occupy a variety of niches as grazers, filter feeders, detritivores, and predators, contributing to nutrient cycling and community structure in marine ecosystems.

Echinoderms have a rich fossil record dating to the early Paleozoic and are important for studying deuterostome evolution. They also serve as model organisms in developmental biology, particularly in studies of embryogenesis and regeneration. Conservation concerns arise from habitat loss, overfishing, and ocean acidification, which can affect their calcareous skeletons and diverse marine habitats.