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

Echinodermata Classes

Echinoidea, urchins
Crinoidea, sea-lilies, feather stars
Asteroidea, starfish
Holothuroidea, sea cucumbers
Ophuroidea, brittle 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 Echinoidea

These are the sea urchins and sand dollars; 950 living species have been described, but over 5000 fossil species have been discovered so far.

E. esculentus (the edible sea-urchin, above right) is the largest and most common species found in U. K. waters, and can be found between the lower shoreline and as deep as 1000 m.

In these animals the five arms have been incorporated into the body (see Echinus esculentus, above right) and the ossicles form a rigid test with pedicellariae and moveable spines. They also have tube feet that aid the spines in locomotion.

The mouth is on the lower surface and has a distinctive chewing apparatus called Aristotle's lantern. It is made up of five large and several small plates, and can be pushed through the mouth. The anus is on the upper surface.

Sea unrchin, Echinus esculentusabove Echinus esculentus, the edible sea urchin
Clypeaster rosaceous, sand dollar, sea biscuit

Clypeaster rosaceous, right aka Clypeaster rosaceus, is an example of the sand dollar or sea biscuit, it is found in the Caribbean. Sand dollars have short spines, and are bilaterally symmetrical.

Paracentrotus lividus, purple sea urchin

Some in this class have become secondarily bilateral (see the sand dollar above right), and there is a general movement of the anus to the posterior end and the mouth to the anterior end.

Echinoidea are found on the bottom of all seas at all depths. Urchins tend to prefer rocky or hard surfaces, whereas sand dollars prefer to burrow in sand.

Reproduction. The sexes are separate and fertilization is external.

On the left is Paracentrotus lividus, the purple sea urchin. It is found in the N. E. Atlantic and the Mediterranean on the lower shore in rock pools and down to a depth of 3 m or more.

It uses its spines to burrow into soft rocks, often limestone, as the depression this creates provides protection from wave action and desiccation at low tide. The depression or burrows increase in size as the urchin grows.

It can reach up to 7 cm in diameter, and is considered a delicacy in some Mediterranean countries. It moves around 4 m per day in an erratic path to graze on algae.

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

Class Asteroidea

These are the starfish or sea stars. There are about 1500 species, and range in size from 1 - 100 cm in diameter.

They have a flattened body with five (sometimes more) fairly broad arms (see right). There is usually an ocellus (primitive eye) at the end of each arm, but their sense organs are not well-developed. The mouth is on the underside and the madreporite and anus on the upper surface. A groove runs from the mouth to the tip of each arm, and rows of tube feet, most with suckers, are situated along either side of the groove.

The water-vascular system opens to the outside through the madreporite. The anus is inconspicuous in most, and a few species lack both anus and intestine.

The skeleton is a loosely organised series of small calcareous plates (ossicles) bound together with connective tissue, and is located beneath the epidermis which has pedicellariae (small forceps, scissors or clamp shaped calcareous structures).

The pedicellariae keep the body surface free of debris, and may function in prey capture; they are also used in identification of species.

Most have separate sexes, but, some Asteroidea are hermaphrodite and are capable of releasing as many as 2.5 million eggs in 2 hours. Reproduction is also possible by fission.

Predatory species may have an eversible stomach. This means the stomach can be inserted into any small hole in the prey's armour and digestion can proceed from there. Bivalves are commonly preyed on in this manner. They are mainly nocturnal. Above right is Plecaster decanus found in southern Australian waters.

Plecaster decanus, star fish

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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