echinozoans

Echinozoans are a subphylum of the phylum Echinodermata, which includes starfish, sea urchins, and brittle stars. This group is characterized by a pentaradial symmetry, meaning their bodies are arranged around a central axis in five parts. Echinozoans are exclusively marine and are found in all oceans worldwide, from shallow waters to the deep sea.

The subphylum Echinozoa is divided into two extant classes: Asteroidea (starfish) and Echinoidea (sea urchins and sand dollars). While starfish are often recognized by their distinctive star shape with multiple arms radiating from a central disk, sea urchins are typically spherical or flattened and lack arms. Both groups share a common ancestor and exhibit a calcareous skeleton, often called a test.

Locomotion in echinozoans is achieved through a unique water vascular system. This hydraulic system consists of a network of canals and tube feet that extend from the body. By controlling the water pressure within these canals, echinozoans can extend, retract, and move their tube feet, allowing them to crawl, grip surfaces, and capture prey. Their diet varies, with some species being predators, scavengers, or detritivores.

Echinozoans play important roles in marine ecosystems, acting as grazers, predators, and bioturbators, influencing the structure and health of benthic communities. Fossil evidence suggests echinozoans have a long evolutionary history, with some of the earliest known fossils dating back to the Ordovician period.