Echinozoa

Echinozoa is a subphylum of the phylum Echinodermata. Echinoderms are marine invertebrates characterized by radial symmetry, a water vascular system, and a calcareous skeleton. The subphylum Echinozoa includes living groups such as sea urchins, sand dollars, and brittle stars, as well as fossil forms. A key characteristic of echinozoans is the absence of arms, which are present in other echinoderm subphyla like Asterozoa (starfish and brittle stars). Instead, echinozoans typically have a globular or flattened, disc-like body.

The body of an echinozoan is generally divided into five distinct sections, reflecting their pentaradial symmetry. This symmetry is evident in the arrangement of their tube feet, which are used for locomotion, feeding, and respiration. Their skeleton, called a test, is composed of fused ossicles, forming a rigid or semi-rigid structure that encloses the internal organs. Echinoderms, including echinozoans, possess a unique water vascular system, a network of fluid-filled canals that operate hydraulic tube feet.

Living echinozoans are primarily benthic, meaning they live on the seafloor, where they can be found in a wide range of marine environments, from shallow coastal waters to the deep sea. Their diet varies, with some species being herbivores, others detritivores, and some carnivores. Fossil evidence indicates that echinozoans have a long evolutionary history, with early forms appearing in the Cambrian period. Their diversification has led to a variety of forms adapted to different ecological niches within marine ecosystems.