Echinoderms and Invertebrate Deuterostomes

Echinoderms, such as sea stars and sea urchins, are spiny-skinned marine deuterostomes with a water-vascular system and secondarily radial adult bodies, sitting on the same great branch as the chordates.

Definition

Echinoderms and invertebrate deuterostomes are non-chordate members of the deuterostome lineage; echinoderms are spiny-skinned marine animals with an internal calcareous skeleton, a water-vascular system, and pentaradial adult symmetry, and they share deuterostome development with hemichordates and chordates.

Scope

This topic covers the Echinodermata, the exclusively marine phylum that includes sea stars, brittle stars, sea urchins, sea cucumbers, and crinoids, and introduces the other invertebrate deuterostomes such as the hemichordates. It treats their distinctive features, including a calcareous endoskeleton, secondarily acquired five-part radial symmetry in adults, and the unique water-vascular system that powers their tube feet, and explains why echinoderms are deuterostomes despite their radial appearance.

Core questions

• What features define echinoderms, and how does their water-vascular system work?
• Why are echinoderms classified as deuterostomes despite their radial adult symmetry?
• How do the major echinoderm classes differ in form and habit?
• What links echinoderms to the chordates within the deuterostomes?

Key theories

Water-vascular system

Echinoderms possess a hydraulic network of canals derived from the coelom that operates rows of tube feet, providing a unique means of locomotion, attachment, feeding, and gas exchange found in no other phylum.

Deuterostome affinity behind secondary radial symmetry

Echinoderm larvae are bilaterally symmetric and develop in the deuterostome manner, so the radial symmetry of the adult is a secondary, derived condition that does not obscure their close relationship to hemichordates and chordates.

Mechanisms

The echinoderm water-vascular system is a set of fluid-filled canals connected to the exterior through a sieve plate; water pressure regulated by muscular ampullae extends and retracts hundreds of tube feet, which the animal uses to creep, grip surfaces, and manipulate food. The endoskeleton consists of calcareous plates or ossicles embedded in the body wall, often bearing spines. Despite the radial adult body, echinoderms share deuterostome development, in which the first embryonic opening becomes the anus and the mouth forms later, and their larvae are bilaterally symmetric, marking the group as relatives of the hemichordates and chordates.

Clinical relevance

Echinoderms are important members of marine ecosystems, where grazing urchins and predatory sea stars can shape entire communities; the sea urchin is a classic model organism for the study of fertilisation and early development. This is educational context, not clinical advice.

History

Echinoderm larvae and development were studied intensively in the nineteenth century, with Johannes Müller describing many larval forms, and the deuterostome grouping of echinoderms, hemichordates, and chordates was established through comparative embryology. Twentieth-century work, including Hyman's treatise and Garstang's ideas on larval evolution, clarified that echinoderm radial symmetry is secondary, a conclusion strongly supported by later molecular phylogenetics.

Key figures

• Johannes Müller
• Libbie Hyman
• Walter Garstang

Seminal works

• pechenik2015
• ruppert2004

Frequently asked questions

Why are sea stars considered relatives of vertebrates?

Echinoderms are deuterostomes, the same major branch of the animal tree that includes the chordates and vertebrates, sharing a pattern of early development that unites the group despite very different adult bodies.

What is the water-vascular system?

It is a hydraulic network of fluid-filled canals unique to echinoderms that operates their tube feet, enabling locomotion, attachment, feeding, and gas exchange.