ECHINODERMATA, sea urchins, sea stars and starfish, sea cucumbers, sea lilies and feather stars



Body shape is highly variable, radial, pentamerous symmetry, bilateral symmetry in some. A distinct head
A mesodermal skeleton of calcareous ossicles and spines Excretory organs
A water vascular system that operates the tube feet and tentacles, with madreporite (opening to exterior) in most Brain
Locomotion by tube feet in most  
Sexes are usually separate  
Larvae bilaterally symmetrical  
Marine; found in all seas at all depths, nearly always on the sea bed.  

Overview of the Phylum Echinodermata

Greek: echinos = hedgehog; derma = skin

The Echinoderms have three features that distinguish them from other phyla:

  • radial symmetry
  • calcareous mesodermal skeleton
  • water vascular system.

The fossil record goes back to the Cambrian. It is believed that the phylum originated from a sessile suspension feeder with a circle of five branching arms similar to a lophophore. And that in free-living ancestors the body axis turned so that the mouth faced downwards, and the arms developed to serve a locomotory function.

It is thought that the ampulla (hydraulic reservoir for each foot) that allows each foot to be individually controlled also developed as the animals became free-living.

In some the calcareous skeleton is a loose association of plates, while in others it has fused to form a rigid test around the animal.

The function of the madreporite is uncertain; it may function in water exchange, or in equalising hydrostatic pressure.

There are over 6000 living species of Echinoderms separated into six classes.

Class Crinoidea

These are the sea-lilies or feather stars, and are thought to retain the ancestral body plan with the upwardly facing mouth. The body consists of a central disk containing the main organs, circled by 1- 200 long, feathery arms. The arms are muco-ciliary and are branched from the five basic arms. Crinoids do not have a madreporite, and the sexes are usually separate. In some species there is a stem anchoring the animal to the substrate. Over time there has been a trend towards free living and away from the anchored forms.

Just over 600 species survive today, but the fossil record shows many more species. They are generally from 15 - 30 cm long, but some fossil species were 20 m long.

They are plankton feeders and are usually found below 100 m, but can also be found in shallow waters. They are particularly abundant in the tropical western Pacific.

Antedon bifia (above left) is the only crinoid found in British waters. It is fragile, so it is usually found in deeper waters away from wave action, often clinging to wrecks.

It is a free living crinoid. The 10 arms form a food-catching funnel, but are also used in locomotion. Sometimes it is found with an annelid worm attached near to its mouth. The two live commensally.

Antedon bifida, a free living crinoid
above Antedon bifia, a sea lily

Japanese sea lily, Metacrinus rotundus

Japanese sea lily, Metacrinus rotunda

The Japanese sea lily, above, is a stalked crinoid found in water 210 - 270 metres deep off the west coast of Japan. In favourable conditions dense beds of sea lilies can be found. Its stalk is strong and made of calcareous ossicles bound by ligaments. The stalk ends in a sucker which anchors it to the bottom. The stalk continues to grow throughout its life, and can reach 40 cm long. It has the power to regenerate both stalk and arms.

When opened up is resembles a feather duster with its mouth at the centre, and its arms spread to gather particulate food carried to it on the water current. The food particles (plankton mainly) are transferred by its tube feet into a groove that runs down its arms. Then the particles are carried down the groove to the mouth by cilia.

Endoxocrinus parrae, right is found in the Caribbean Endoxocrinus parrae, Sea lily

Class Holothuroidea

Holothuna forskali, the cotton spinner, sea cucumber

These are the sea cucumbers, Cucumaria sp. below right and Holothuna forskali (the cotton spinner) above.

There are just over 1100 living species, and they have an elongated bilaterally symmetrical soft body, and can grow to 50 cm long.

The skeleton has been reduced to small ossicles. The body muscles are attached to the ring of larger ossicles around the first part of the gut.

They have from 8 - 30 tentacles around the mouth operated by the water vascular system; these are used in feeding. They have a through gut.

Locomotion is mainly by tube feet, but in those species lacking tube feet, by peristaltic muscular contractions.

Respiration in most is via the anus. Water is drawn into special organs called respiratory trees. In small/thin species gaseous exchange takes place through the body wall.

Nutrition. Sea cucumbers live off the decaying matter found in the sand and mud they eat. Those species with longer tentacles also catch small organisms which stick to the slime covering the tentacles.

They are found mostly in or on the sea bed. Holothuna forskali is found in the N.E. Atlantic and Mediterranean, where it crawls on the sea floor like a slug using its tentacles to push mud into its mouth. It lives off the organic particles in the mud.

Defense. When disturbed or feeling threatened sea cucumbers shoot the tube of their respiratory system out of their anus. In extreme circumstances they can even shoot out their whole digestive tract. The tubes are sticky and entangle the potential predator allowing the sea cucumber to escape. The tubes and digestive tract can be re grown in weeks. When Holothuna forskali is threatened it moves its rear end in the direction of the aggressor and shoots out its respiratory trees through its anus. In contact with the water the respiratory trees swell up forming a sticky mass in which the aggressor gets entangled. New respiratory trees are soon regenerated.

Reproduction. The sexes are usually separate, though there are some hermaphrodite species, and fertilisation is external.

Curcumaria, sea cucumber
above Cucumaria sp., a sea cucumber
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