Echinodermata Classes

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

Class Ophuroidea

These are the brittle stars, right and below. They bear a resemblance to the starfish.

There are about 2000 species, found on the sea bottom at all depths. However, unlike starfish these are neither scavengers nor suspension feeders. T

hey are flat with a central disc and (usually) five long, thin arms, see left.

The ossicles of the skeleton are fused, and the arms are used in locomotion. The tube feet are used in feeding. They have no anus or pedicellariae and the madreporite is located on the under surface. The mouth has five jaw-like plates.

They are negatively phototropic, i.e., they move away from light and tend to be more active at night.

Reproduction. The sexes are usually separate, but a few species are hermaphrodites, and asexual reproduction by disc cleavage is possible.

Brittle star

brittlestar

On the left is Lapworthia miltoni, a fossil brittle star form the late Silurian, (423 - 419 million years ago). It was found in what is now Herefordshire, England.

 

Lapworthia miltoni, fossil brittle star

Class Concentricyloidea

These were discovered in 1986, and only two species have been described so far.

They are flattened, circular, with a row of spines around the circumference, and measure about 2 - 9 mm in diameter.

They have tube feet around the edge of the disc, and their upper surface is covered in scale-like plates. However they have no mouth, anus or arms.

So far they have been found in deep water around New Zealand. Superficially they look like a small jellyfish (see right).

Sea dasiy, concentricyloidea
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