Phylum Echinodermata
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Phylum Echinodermata Review

1. What are some representatives of the echinoderm phylum?

Starfishes, sea cucumbers (holothurians), sea urchins and brittle stars are examples of echinoderms.


Phylum Echinodermata - Image Diversity: echinoderms starfish sea cucumber sea urchin brittle star

Echinoderms are marine animals, they live in salt water.

Echinoderms, as the name indicates (echino = spiny, derma = skin), are creatures with spines originated from an endoskeleton. Their endoskeleton is made of calcareous plaques that besides spines contain pedicellaria, small pincers used to clean the body and to help the capturing of prey. They also present a hydrovascular system known as the ambulacral system. Adult echinoderms have pentaradial symmetry; the radial symmetry in these animals is secondary, present only in adults.

The echinoderm skeleton is internal, i.e., it is an endoskeleton. It is made of calcium carbonate (calcareous).

Vertebrates also have internal skeleton made of bones and cartilages. Arthropods have an external carapace made of chitin, a chitinous exoskeleton. Some molluscs present a calcareous shell that works as exoskeleton.

The system that permits movement and fixation to substrates in echinoderms is called the ambulacral system. In these animals water enters through a structure called madreporite, passes through channels and reaches the ambulacral feet in the undersurface of the body. In the ambulacral region in contact with the substrate, there are tube feet filled and emptied by water thus acting as suckers.

Phylum Echinodermata - Image Diversity: ambulacral system

Echinoderms present a complete digestive system, with mouth and anus.

Sea urchins have a teeth-like structure juxtaposed to the mouth and made of five teeth connected to ossicles and muscle fibers. This structure, known as Aristotle’s lantern, is use to scratch food, mainly algae, from marine rocks.

Echinoderms and chordates are deuterostomes, i.e., in their embryonic development the blastopore turns into the anus. All other animals with complete digestive system are protostomes, i.e., their blastopore originates the mouth.

The blastopore is the first opening of the digestive tube that appears in the embryonic development.

Phylum Echinodermata - Image Diversity: blastopore

In echinoderms there are not well-defined respiratory (with the exception of the holothurian group) and circulatory systems. The ambulacral hydrovascular system plays the function of these systems.

Echinoderms do not have an excretory system. Their excretions are eliminated by diffusion.

Adult echinoderms, along with cnidarians, are the animals that present radial symmetry, i.e., their body structures are distributed around a center. The radial symmetry in echinoderms however is a secondary radial symmetry, since their larval stage has bilateral symmetry and the radial pattern appears only in adult individuals (there are some few adult echinoderms with lateral symmetry). All other animals have lateral symmetry with exception of poriferans (no symmetry is defined for them).

Echinoderms do not present cephalization and they have a diffuse network of nerves and neurons made of a neural ring around the mouth and of radial nerves that ramify to follow the pentaradial structure of the body.

Phylum Echinodermata - Image Diversity: radial symmetry

The fecundation in echinoderms is external, gametes are liberated in water where fecundation occurs.

The majority of echinoderms are dioecious with male and female individuals.

In echinoderms embryonic development is indirect, with ciliated larvae.

The five echinoderm classes are: asteroids (starfishes), ophiuroids, crinoids, holothuroids (sea cucumbers) and echinoids (sea urchins and sand dollars).

Examples of representing beings: sea cucumber, sea urchin, starfishes. Basic morphology: calcareous endoskeleton with spines, ambulacral system. Type of symmetry: secondary radial. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete, deuterostomes. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: simple, nerve network without ganglia or cephalization. Types of reproduction: sexual, with larval stage.

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