Echinozoa

Echinozoa is a subphylum of echinoderms characterized by their generally globular or discoidal shape and the absence of arms or rays extending directly from the central disc. This subphylum includes sea urchins, sand dollars, sea cucumbers, sea lilies (crinoids - which exhibit a derived sessile existence with arms radiating upwards), and several extinct classes.

General Characteristics:

• Body Shape: Primarily spherical, ovoid, or disc-shaped. This contrasts with the more radial symmetry seen in other echinoderm groups like asteroids (sea stars) and ophiuroids (brittle stars).

• Absence of Distinct Arms: Although crinoids possess arms, these are generally considered extensions of the calyx (the body portion) and not directly analogous to the arms of asteroids. Other echinozoans lack prominent arms altogether.

• Water Vascular System: Possess a water vascular system, a characteristic feature of all echinoderms, used for locomotion, respiration, and food gathering.

• Endoskeleton: Have an internal skeleton (endoskeleton) composed of calcareous ossicles. In sea urchins and sand dollars, these ossicles are fused to form a rigid test. In sea cucumbers, the ossicles are reduced to microscopic spicules embedded in a leathery body wall.

• Tube Feet: Possess tube feet, which are extensions of the water vascular system used for locomotion, attachment, and feeding.

• Radial Symmetry: While appearing bilaterally symmetrical in some cases (especially sand dollars), they retain a modified form of the pentaradial symmetry characteristic of echinoderms.

Major Classes within Echinozoa:

• Echinoidea (Sea Urchins and Sand Dollars): Characterized by their globular or flattened bodies enclosed in a test composed of fused ossicles. Sea urchins are typically spherical, while sand dollars are flattened and adapted for burrowing.

• Holothuroidea (Sea Cucumbers): Elongated, soft-bodied echinoderms lacking a rigid test. The ossicles are reduced to microscopic spicules. They typically lie on the sea floor and feed on detritus.

• Crinoidea (Sea Lilies and Feather Stars): Sea lilies are stalked and sessile, while feather stars are free-swimming or attached by cirri. They possess a calyx (body) and arms used for filter feeding. Although technically possessing arms, their morphology and evolutionary origin differentiate them from the arms of asteroids and ophiuroids.

• Edrioasteroidea (Extinct): An extinct class of early echinozoans with a flattened or globular body.

• Ophiocistioidea (Extinct): Another extinct class of echinozoans of uncertain affinity.

Ecology and Importance:

Echinozoans play important roles in marine ecosystems. Sea urchins are significant grazers, influencing algal communities. Sea cucumbers are detritivores, contributing to nutrient cycling. Some echinozoans are commercially harvested for food (e.g., sea urchin gonads, sea cucumbers). Fossil echinozoans are also important for understanding the evolution of echinoderms and for biostratigraphy.