Echinodermata: Starfish and Sea Urchins Characteristics

Echinodermata is a phylum of marine invertebrate animals that includes starfish, sea urchins, sea cucumbers, brittle stars and feather stars. The name comes from the Greek words "echinos" meaning spiny and "derma" meaning skin, because most echinoderms have a hard, spiny body covering. Every echinoderm lives in the sea — not a single species lives in fresh water or on land.

They matter because they show one of the strangest body plans in the animal kingdom. Their larvae swim with bilateral symmetry like most animals, but the adults transform into a five-sided radial symmetry found nowhere else. Studying this switch helps scientists understand how animal body plans evolved over millions of years.

Key Concepts

• Exclusively marine: all 7,000+ living species live in oceans, from tidal pools to the deep sea.
• Triploblastic coelomates: the body develops from three germ layers and contains a true coelom formed by enterocoely.
• Pentamerous radial symmetry: adult body parts are arranged in fives around a central disc — five arms, five rows of tube feet.
• Water vascular system: a network of water-filled canals used for locomotion, feeding, respiration and excretion.
• Calcareous endoskeleton: an internal skeleton of calcium carbonate plates called ossicles; sea urchins also carry movable spines.
• No head or brain: the nervous system is a nerve ring around the mouth with radial nerves; sexes are separate and fertilisation is external in seawater.

How It Works

1. Seawater enters through the madreporite, a sieve-like plate on the upper surface of the body.
2. Water flows down the stone canal into the ring canal that circles the mouth.
3. From the ring canal, five radial canals run outward, one into each arm.
4. Each radial canal connects to hundreds of tube feet, and every tube foot has a bulb-like ampulla at its base.
5. When muscles squeeze an ampulla, water pressure extends the tube foot; when they relax, the foot retracts. This hydraulic stepping lets the animal crawl and grip prey such as clams.

Worked Example

A student examines a starfish and wants to predict its canal counts from its symmetry. The starfish has 5 arms.

1. Radial canals = one per arm = 5
2. Main canals = 1 ring canal + 5 radial canals = 6
3. Each arm carries 2 rows of tube feet, so tube-foot rows = 5 arms x 2 = 10

One look at the five-fold symmetry gives all the counts: 5 arms, 5 radial canals and 10 tube-foot rows.

Common Misconceptions

• "Starfish are fish." They are not fish at all — they have no backbone, no gills and no fins. They are invertebrate echinoderms.
• "All sea urchin spines are venomous." Most spines are only sharp for defence; venom occurs in just a few species.
• "A starfish regrows its whole body from any single arm." Regeneration of a full animal needs part of the central disc; a lone arm without disc tissue dies.

Real-World Applications

• Marine ecologists use starfish as indicator species to judge ocean health and study food webs.
• Sea urchin embryos are a classic model in developmental biology for studying fertilisation and early cell division.
• Sea cucumbers are harvested as food (beche-de-mer) in several Asian countries and support coastal fisheries.

Echinoderms are spiny-skinned marine animals with five-fold symmetry, a hydraulic water vascular system and a calcareous skeleton. Their combination of bilateral larvae and radial adults makes them a key group for understanding how animal body plans evolved.