Echinoderms and Hemichordates

Core Concepts

Echinodermata: Defining Characteristics

Deuterostome development. Echinoderms exhibit radial cleavage, indeterminate development, and enterocoelous coelom formation — the deuterostome pattern shared with chordates. This is the single most important phylogenetic fact about echinoderms: they are more closely related to vertebrates than to any protostome phylum, despite their radically different adult body plan.

Endoskeleton. Echinoderms have an internal skeleton composed of calcium carbonate plates (ossicles) covered by a thin epidermis. In sea urchins, the ossicles are fused into a rigid test. In sea stars, they are loosely articulated, allowing flexibility. In sea cucumbers, they are microscopic and scattered. The endoskeleton provides support and protection without the molting requirement of an exoskeleton.

Water vascular system. A unique hydraulic system that operates tube feet. The components:

• Madreporite: A sieve-like opening on the aboral (top) surface through which seawater enters.

• Stone canal: Connects the madreporite to the ring canal.

• Ring canal: Encircles the mouth.

• Radial canals: Extend from the ring canal into each arm (in sea stars) or body section.

• Tube feet (podia): Extensions of the radial canals. Each tube foot has an internal bulb (ampulla) and an external podium. Contraction of the ampulla extends the tube foot by hydrostatic pressure. Suction at the tip allows attachment to surfaces.

The water vascular system functions in locomotion, feeding (prying open bivalves in sea stars), gas exchange, and sensory perception. It is entirely hydraulic — powered by muscular contraction and water pressure, not by a pump.

Adult radial symmetry (pentaradial). Adult echinoderms have five-part symmetry — body parts arranged in fives or multiples of five around a central axis. This is a derived condition: echinoderm larvae are bilaterally symmetrical, and molecular evidence confirms that echinoderms evolved from a bilaterally symmetrical ancestor. The radial symmetry evolved secondarily, likely as an adaptation to a sessile or slow-moving benthic lifestyle, where encountering the environment equally from all directions is advantageous.

Mutable connective tissue. Echinoderms can rapidly and reversibly change the stiffness of their connective tissue — from flaccid to rigid — under nervous control. This allows a sea star to hold a rigid position for hours while prying open a bivalve without muscular fatigue.

Regeneration. Many echinoderms have remarkable regenerative abilities. Sea stars can regenerate lost arms; some species can regenerate an entire body from a single arm if it includes part of the central disc. Sea cucumbers can eviscerate — expel their internal organs — as a defense mechanism and regenerate them within weeks.

Major Echinoderm Classes

• Asteroidea (sea stars): Five or more arms, not sharply demarcated from the central disc. Tube feet on the oral (bottom) surface. Predatory — many evert their stomach into bivalves and digest prey externally.

• Ophiuroidea (brittle stars): Five slender, sharply demarcated arms. Move rapidly by arm movement. Tube feet lack suckers and are used primarily for feeding, not locomotion.

• Echinoidea (sea urchins, sand dollars): No arms. Ossicles fused into a rigid test. Movable spines for protection and (in some) locomotion. Aristotle’s lantern — a complex jaw apparatus — grazes algae.

• Holothuroidea (sea cucumbers): Elongated, soft-bodied — ossicles reduced. Oral tentacles (modified tube feet) collect food. Respiratory trees (branched tubes in the cloaca) for gas exchange. Evisceration defense.

• Crinoidea (sea lilies, feather stars): The most ancient class. Oral surface faces upward. Feathery arms with tube feet for filter feeding. Sea lilies are attached by a stalk. Feather stars are free-swimming as adults.

Hemichordata: Acorn Worms

Hemichordates are marine, worm-like deuterostomes. The most familiar group is the enteropneusts (acorn worms), which burrow in marine sediments and deposit-feed. Key features:

• Pharyngeal slits: Openings in the pharynx used for filter feeding (not respiration, as in aquatic chordates). These slits are homologous to chordate pharyngeal slits and are a key piece of evidence for the deuterostome relationship.

• Stomochord: A short, stiff rod in the anterior body that was once considered homologous to the notochord but is now understood to be a different structure — likely an anterior extension of the gut.

• Dorsal nerve cord (partial): Some hemichordates have a short, hollow dorsal nerve cord in the collar region — possibly homologous to the chordate dorsal hollow nerve cord — alongside the main ventral nerve cord.

• Three-part body: Proboscis, collar, and trunk.

Hemichordates are not chordates, but they share features that illuminate the evolutionary assembly of the chordate body plan. Their phylogenetic position — sister group to echinoderms, or close to the chordate lineage — continues to be refined.

Pterobranchs are a smaller, colonial group of hemichordates that live in secreted tubes and feed with ciliated tentacles.

Evolutionary Connection

Echinoderms and chordates share a deuterostome common ancestor. This means that the echinoderm body plan — with its radial symmetry, water vascular system, and mutable connective tissue — evolved from a bilaterally symmetrical ancestor, representing one of the most dramatic evolutionary transformations in the animal kingdom. The hemichordate body plan, with its pharyngeal slits and partial dorsal nerve cord, may approximate the ancestral deuterostome condition from which both the echinoderm and chordate lineages diverged. The shared deuterostome developmental program — radial cleavage, indeterminate development, enterocoely — was inherited from this common ancestor and is conserved in both echinoderms and chordates despite their radically different adult morphologies.

ELI-10

Echinoderms are the spiny-skinned animals — sea stars, sea urchins, sea cucumbers. And here is the surprising thing: they are more closely related to us (vertebrates) than they are to insects, worms, or mollusks. They share a pattern of embryo development (deuterostome) with chordates, which is a stronger indicator of relationship than adult appearance.

The echinoderm body is built on a five-part plan, like a star. But their larvae are bilaterally symmetrical with left and right sides — the radial adult body is a later modification, not the ancestral state.

Their most unusual feature is the water vascular system — a network of water-filled canals that powers hundreds of tiny tube feet. A sea star extends its tube feet by squeezing water into them (hydraulic pressure), attaches the tips to a surface with suction, and pulls itself along. To open a clam, a sea star wraps its arms around the shell, attaches tube feet, and pulls steadily — the clam eventually tires and gapes open. Then the sea star does something wild: it pushes its stomach out through its mouth, into the clam, and digests the clam’s body externally. After the meal, it pulls its stomach back in.

Sea cucumbers can spit out their internal organs when threatened — a defense called evisceration — and grow them back in a few weeks. Sea stars can regenerate lost arms from almost nothing.

Acorn worms (hemichordates) are less flashy but important: they are worm-like creatures with gill slits and a partial nerve cord along the back, hinting at the features that would eventually become the chordate body plan.

ELI Example

A sea star is a hydraulic robot with a five-part body plan. Its tube feet are like tiny water balloons on the end of hydraulic lines — squeeze water into the balloon, it extends; let water out, it retracts. The whole system runs on seawater and muscle power. It is slow but relentless: a sea star can hold a clam under steady tension for hours, and the clam’s muscles eventually exhaust. Then the sea star turns its stomach inside out (imagine sticking your stomach out through your mouth to digest food on your plate) and eats the clam right inside its own shell.

Do Not Confuse

• Echinoderm vs. Cnidarian (radial symmetry): Both have radial symmetry as adults, but echinoderms are triploblastic, coelomate, deuterostomes, with an endoskeleton and water vascular system. Cnidarians are diploblastic, have a gastrovascular cavity, cnidocytes, and a nerve net. The radial symmetry is convergent, not homologous.

• Water Vascular System vs. Circulatory System: The water vascular system is a unique hydraulic system for locomotion, feeding, and gas exchange. It is separate from the reduced circulatory system (hemal system). It is not the echinoderm equivalent of blood vessels.

Lab Link

When observing a sea star in the laboratory, identify the oral and aboral surfaces, the madreporite (small, off-center disc on the aboral surface), and the tube feet in the ambulacral grooves on the oral surface. Examine the water vascular system in a preserved or injected specimen. Observe a sea urchin’s test (skeleton) — note the five-part symmetry, the tubercles where spines attach, and the holes for tube feet. Compare a brittle star’s sharply demarcated arms with a sea star’s gradual arms.

High-Yield Memory Anchors

• Echinoderm = deuterostome, endoskeleton (CaCO3 ossicles), water vascular system, tube feet, pentaradial adult symmetry, mutable connective tissue.

• Sea star (Asteroidea), brittle star (Ophiuroidea), sea urchin (Echinoidea), sea cucumber (Holothuroidea), crinoid (Crinoidea).

• Water vascular system: madreporite → stone canal → ring canal → radial canals → tube feet.

• Hemichordate = pharyngeal slits + (partial) dorsal nerve cord + stomochord. Deuterostome.

Quick Check

Q1: The water vascular system of echinoderms functions in all of the following EXCEPT:

A) Locomotion

B) Feeding

C) Transport of oxygen by hemoglobin

D) Gas exchange

Q2: An adult sea star has radial symmetry, but its larva is bilaterally symmetrical. What does this indicate about the evolutionary history of echinoderms?

Q3: Compare the pharyngeal slits of hemichordates and chordates. How does the similarity support a shared deuterostome ancestry, and how do the functions differ?

Quick Check Answers

A1: C. Transport of oxygen by hemoglobin. The water vascular system operates by hydraulics using seawater. It is not a blood-based oxygen transport system. Echinoderms have a separate, reduced hemal system for circulation, and gas exchange occurs across tube feet and (in sea stars) dermal gills (papulae).

A2: The bilateral larva and radial adult indicate that radial symmetry in echinoderms is a derived (secondarily evolved) condition, not the ancestral state. The common ancestor of echinoderms and other deuterostomes was bilaterally symmetrical. The radial adult body plan evolved as an adaptation to a benthic, slow-moving or sessile lifestyle, where encountering food, predators, and sensory information from all directions equally is advantageous. This is supported by molecular phylogeny, which confirms echinoderms are nested within the Bilateria.

A3: Both hemichordates and chordates have pharyngeal slits — openings in the pharynx that connect the pharyngeal cavity to the outside. The shared presence of this structure is evidence of common deuterostome ancestry (homology). The function differs: in hemichordates (acorn worms), pharyngeal slits are used for filter feeding — water enters the mouth, food particles are trapped in mucus, and water exits through the slits. In aquatic chordates (e.g., fish), pharyngeal slits are used for respiration — water passes over gills in the slits, and gas exchange occurs. The structure (pharyngeal slits) is shared due to common ancestry; the function (filter feeding vs. respiration) diverged in different lineages.

Chapter Summary

Echinoderms are marine deuterostomes with a calcium-carbonate endoskeleton, a unique water vascular system powering tube feet, adult pentaradial symmetry (derived from a bilateral ancestor), and remarkable regenerative abilities. Major classes include sea stars, brittle stars, sea urchins, sea cucumbers, and crinoids. Hemichordates (acorn worms) are worm-like deuterostomes with pharyngeal slits and partial dorsal nerve cords that illuminate the evolutionary assembly of chordate features. The shared deuterostome developmental program unites echinoderms, hemichordates, and chordates.

Common Mistakes

• “Sea stars are fish.” Sea stars are echinoderms — invertebrates. The name “starfish” is misleading; “sea star” is preferred.

• “Echinoderms are ‘primitive’ because they have radial symmetry.” Radial symmetry in echinoderms is derived, not ancestral. Their larvae are bilateral. The radial adult body plan evolved from a bilateral ancestor. Complexity and derived traits are not synonyms for “advanced,” and simple-looking body plans are not necessarily ancestral.