echinos

Echinos refers to echinoids, a group of echinoderms that includes sea urchins, sand dollars, and heart urchins. They possess a calcareous endoskeleton called a test, made of fused plates and usually covered with movable spines. Most have a compact, globe- or disk-shaped body and use tube feet for movement and feeding.

Anatomy and feeding: Echinoids show fivefold radial symmetry and lack arms. The mouth sits on the underside within Aristotle’s lantern, a jaw-like structure used to graze algae. Spines and pedicellariae aid in defense, cleaning, and sensation.

Habitat and ecology: They inhabit oceans worldwide, from intertidal zones to the deep sea, in reefs, kelp beds, and soft sediments. They play important roles as grazers and as prey for many predators. Irregular echinoids such as heart urchins and sand dollars often burrow or lie flat in sediment.

Reproduction and development: Most species reproduce sexually with external fertilization; embryos develop into free-swimming larvae (such as echinopluteus) before metamorphosis into juveniles. Some taxa show direct development. Life cycles and larval forms vary across groups.

Taxonomy and fossil record: Echinoidea comprises regular and irregular echinoids, with diverse orders. The fossil record extends back to the Ordovician, making echinoids important for studies of marine history. Some species are harvested for food, and many face pressures from habitat loss and ocean acidification that affect their calcium carbonate tests.