Echinoderm

**Echinoderms**  are a successful phylum of marine animals. They include sea stars, brittle stars, sea urchins, sea cucumbers and their relatives.

Echinoderms have these features:
1. A skeleton of plates. These are formed from calcite, a mineral made of calcium carbonate. The plates are usually spiny, and the skeleton is covered outside and in by a layer of skin.
1. Five-rayed (pentameral) symmetry.
1. A water-vascular system. This is an internal system of tubes and bladders filled with water.
1. Tube feet. These are extensions of the water-vascular system. The tube feet poke out from the skeleton, and are used in walking, respiration and feeding. The animal handles the world outside by using its tube feet.
1. Stenohaline. They can't handle big changes in the salinity of the water. No doubt this is why they are:
1. Entirely marine.

Echinoderms live in all parts of the ocean, but mostly on the sea floor. Some are filter feeders, and others (starfish) are important predators of molluscs and other shell-fish. They are extremely common near the shore, and on reefs.

They have a long and abundant fossil record. This phylum appeared in the early Cambrian period; it contains about 7,000 living  and 13,000 extinct species. The four or five main groups are called sub-phyla by some authorities, and classes by others.

Echinodermata is the largest animal phylum which is entirely marine: no animals in this group live on land or in fresh or brackish water.

### A modern classification

- Asterozoa: about 1,500 species that catch prey for their food.
    - Asteroidea: the starfish.
        - Sea daisies, notable for their unusual water vascular system. Only two species, now merged into Asteroidea.
    - Ophiuroidea (brittle stars and basket stars), the largest echinoderms; about 1,500 species.
- Crinozoa (crinoids: the feather stars or sea lilies): about 600 species that are suspension feeders. Originally these were stalked echinoderms with long arms, rather plant-like in appearance. In this form they are called 'sea lilies'. The P/Tr extinction event almost eliminated them. Now they are reduced to a few deep-water species. The free-living 'feather stars' in shallow water are quite successful today.
- Blastozoa: the cystoids, an extinct group. They were mostly sessile and lived in shallow waters. Sometimes put together with crinoids in the subphylum Pelmatozoa.
- Echinozoa: echinoids and sea cucumbers.
    - Echinoidea: sea urchins and sand dollars, notable for their movable spines. There are about 1,000 species.
    - Holothuroidea (sea cucumbers). Relatively soft-bodied animals with a tube-like shape; about 1,500 species. The usual calcareous elements in the skin are much reduced, but the skin is quite tough, and covered in slime. They have good defences against predators.
- Homalozoa: an extinct group of strange early echinoderms. Some put them in the phylum Chordata, and call them calcichordates.

### A simplified classification

For many purposes it is better to use the following classification. It is easier to learn and understand, and is based more on the living forms.<sup>p19</sup>

1. Asteroidea: the starfish.
1. Ophiuroidea: the brittle-stars
1. Echinoidea: the sea-urchins
1. Holothuroidea: the sea-cucumbers
1. Crinoidea: the sea-lilies and feather-stars.

Only with the crinoids is it necessary to deal with the fossil forms, because the stalked crinoids were so important on the Palaeozoic sea floors.

## General account

All echinoderms have fivefold radial symmetry in their body at some stage of life. The holothurians have bilateral symmetry as adults, and do not have so much of a skeleton as other groups.

Echinoderms have a hydraulic water vascular system. This network of fluid-filled canals moves and feeds the animal, and allows gas exchange. There is also a complete digestive tube. They have a simple radial nervous system that consists of a modified nerve net. There are nerve rings with radiating nerves around the mouth extending into each arm. The branches of these nerves coordinate the movements of the animal. Echinoderms have no brain, although some do have ganglia. They usually move with their tube feet, but brittle stars use their arms to pull against rocks. Some feather stars and sea cucumbers can swim.

The sexes are usually separate. Sexual reproduction typically consists of releasing eggs and sperm into the water, with fertilization taking place externally. Their larvae are planktonic.

Many echinoderms have remarkable powers of regeneration. A starfish arm cut off will regenerate. A section as small as a single arm, with some central disk and neural tissue, can regenerate the whole organism.

### Echinoderm larvae

Most of the adults have radial symmetry. If they are not sessile or attached, they are at least benthic (bottom-dwellers). In contrast, echinoderm larvae are planktonic, and have bilateral symmetry.<sup>p119</sup> Echinoderm larvae are ciliated free-swimming organisms that have a bilaterally symmetry rather like embryonic chordates. The first stage in all groups except the crinoids is the **dipleurula**, which has a row of cilia winding round its body. All the groups have second and third stage larvae. The larvae grow, and are carried away from their parents.

Later, the adult body grows into a radial form, with the body arranged in five parts around a central axis. This means they often have a star-shaped pattern.

## Evolution

There is no geological evidence about the origin of echinoderms. The first undoubted fossils in the Lower Cambrian are already well developed, and fall into five or six different groups. _Arkarua_, from the Ediacaran, is thought to be an echinoderm. _Tribrachidium_ is another possible, from the same period. On general biological grounds, it seems that echinoderms, and some related phyla, may have been derived from worm-like ancestors.<sup>p311</sup>

Of all invertebrate phyla, the echinoderms are the closest relatives to the chordata, that is, animals who have spinal cord, such as vertebrates. With the chordates and hemichordates, echinoderms are deuterostomes, one of the two major divisions of the animal kingdom.

## References

- Clark A.M. 1968. _Starfishes and their relations_, 2nd edition. British Museum (Natural History).
- Paul C.R.C & Smith A.B. (eds) 1988. _Echinoderm phylogeny and evolutionary biology_. Oxford.

## Other websites

- Echinodermata  from the _Tree of Life_ website.