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. They are flat with a central disc and (usually) five long, thin arms, see left.
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 feed on microscopic plankton, both plant and animal organic particles, and some feed on larger items such as bristleworms or small crustaceans.
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.