Echinoderms - Echinodermata - Who are Echinoderms and how do they live


There are several animals that we find in the phylum Echinodermata, some very well known, others unknown to most. The ones that are most commonly found and that we are all familiar with are the starfish (Asteroidea), sea urchins and sand dollars (Echinoidea), sea cucumbers or sea cucumbers (Holothuroidea), the ophiurias (Ophiuroidea) and sea lilies (Crinoid).

The name echinoderms derives from the Greek and precisely from echinos "Hedgehog" edermis "Skin" to show how often their epidermis is covered with various formations such as spikes or other protuberances.



The approximately 7000 species of animals that we find in this phylum are very important in the marine ecosystem and of which there are many fossils datable to the Mesozoic, Cenozoic and Paleozoic periods.

They are exclusively marine animals and so far no fossil evidence has been found that their life, in some past era, also took place on land or in fresh water.

In the marine environment they occupy all habitats, none excluded, representing an important part of biomass. They have also been found at the greatest marine depths explored so far.


We find animals with the most bizarre shapes and with completely unique characteristics in the animal kingdom. However, they are easily identifiable if we consider some peculiar characteristics:

  • they have an endoskeleton made up of calcium carbonate and different proteins organized in calcareous plates or calcareous ossicles, which can be fixed or mobile and covered by an epidermis often provided with spines or protuberances of various kinds;
  • the skeletal system is held together by a "mutable" connective tissue (defined as variable tensility) which now allows to have a rigid, now softer structure allowing the animal to assume particular positions without any physical effort;
  • adults are characterized by a radial symmetry that manifests itself with multiple radial divisions of five, that is to say that usually their body is divided into five parts (or multiples thereof);
  • they are characterized by an aquifer system that has two functions: walking and capturing food particles. The movement takes place thanks to the transport of water inside special channels (as they lack a muscular system) and works on the basis of the principle of osmotic pressure (when a solvent passes from a more concentrated solution to another less concentrated) and this does yes that they are animals that can only live in salt water.

Having an endoskeleton made of calcite makes these animals important in the marine ecosystem as they contribute to the formation of calcium deposits.


Echinoderms are animals with separate sexes (with some rare exceptions) and generally do not present sexual dimorphism.


Their diet is different: some echinoderms are in fact considered real predators actively hunting their preys (for example the sea stars are fearsome predators); others feed on organic particles dissolved in water; others of algae.

Monographic sheets of single species of echinoderms

Video: 20. Echinoderm Development: Fertilization and Cleavage Dendraster eccentrius

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